Improve Deliverability for Customer.io & Braze

Why are your automated emails from Customer.io and Braze not reaching inboxes?

You’ve built a flawless journey in Customer.io or Braze — triggered by behavior, personal, on time. But open rates are flat. Delivery reports show hard bounces. Your inbox placement is stuck in the 60s. And you're asking: why?

It’s not the message. It’s not the timing. The bottleneck is often not in the platform — it’s in the list. Invalid, role-based, or disposable email addresses don’t just fail to open; they damage your sender reputation, increase bounce rates, and trigger spam filters. Even the most well-crafted automation fails if the foundation is broken.

Improving deliverability rates for Customer.io and Braze automated journeys starts with fixing your list before the first send. A single bad address can cost you inbox access.

Key takeaways

Invalid or disposable email addresses in your Customer.io or Braze journey can cause hard bounces and hurt sender reputation.
Role accounts (like admin@ or sales@) are often unreliable and increase spam likelihood.
Proactively validating emails before automation ensures higher inbox placement and reduces delivery risks.

How does list hygiene directly improve deliverability for automation platforms?

You improve deliverability for Customer.io and Braze journeys by removing invalid, disposable, or non-existent email addresses before sending. Every hard bounce signals to inbox providers that your list is unreliable. High bounce rates reduce inbox placement, even for automated campaigns. Cleaning your list proactively prevents reputation damage and keeps your messages in inboxes, not spam folders.

Hard bounces hurt sender reputation fast

When you send to an email address that doesn’t exist, the receiving server returns a hard bounce. Inbox providers like Gmail and Outlook track these failures. A single bounce might not trigger action, but consistently high rates do. According to a report by Return Path, senders with hard bounce rates above 2% are significantly more likely to be filtered or blocked.

Automated journeys, especially those triggered by user behavior (like welcome series or cart abandonment), rely on clean, accurate data. Sending to invalid addresses wastes sender reputation and inflates your bounce rate unnecessarily. These automated flows often run at scale, making even low bounce rates problematic if left unchecked.

Bounce rates correlate with inbox placement

Mailchimp’s published benchmarks show that campaigns with bounce rates under 2% have consistent inbox placement. Rates above 5% often trigger deliverability warnings or reduced delivery windows. Because automation platforms like Customer.io and Braze send to thousands or millions of contacts in real time, the cumulative impact of poor hygiene can be rapid and damaging.

Disposable email addresses and role accounts (like admin@ or support@) further degrade results. They’re often used for fake sign-ups, ignored, or deliberately flagged. Even if they don’t bounce, they don’t convert and can still harm your sender reputation over time. According to Spamhaus, high volumes of messages to disposable domains can correlate with spam signals.

Let’s be clear: no automation software can fix a dirty list. Tools like MailTester’s bulk verification catch hard bounces, invalid syntax, and catch-all addresses before you send. Real-time checks via the email verification API integrate directly into your lead capture or onboarding workflows, ensuring only valid addresses enter your automation pipelines.

Improving deliverability starts with knowing your list’s true quality. The fewer bounces, the better your sender reputation—and the more likely your next automated campaign lands in the inbox, not the trash.

What types of email addresses should be removed before launching Customer.io or Braze journeys?

You should remove invalid addresses (invalid syntax, non-existent domains), role-based emails (admin@, support@, sales@), disposable emails (like TempMail, Mailinator), catch-all domains (where any address is accepted), and known spam traps or outdated aliases. These types increase bounce rates, harm sender reputation, and reduce inbox placement—especially in automated journeys where clean data is critical. Let’s break down why each matters.

Invalid and non-deliverable addresses

These include emails with malformed syntax (like user@@domain.com) or domains that don’t exist. Sending to these generates hard bounces and wastes send capacity. According to RFC 5321, SMTP rejects messages to non-routable domains during delivery. Tools like MailTester’s email checker validate syntax and domain existence in real time.

High-risk sender addresses

Role-based emails (admin@, sales@, info@) often belong to shared inboxes. They’re easily abused, have high bounce rates, and are frequently flagged as low quality by inbox providers. A 2021 study by Return Path found that role-based addresses have a bounce rate 3x higher than personal addresses on average. Removing them protects your sender reputation.

Disposable and temporary domains

Services like Mailinator, TempMail, or 10MinuteMail create temporary inboxes. Users typically don’t engage with automated journeys. These domains are flagged by most ESPs and can trigger spam filters. High volumes from disposable domains signal low-quality list management, which can lead to IP blacklisting. You don’t want these cluttering your automation pipelines.

Catch-all domains and spam traps

Catch-all domains accept any email address, even if it doesn’t exist. While this seems beneficial, it allows spammers to test email lists. If you send to these, your messages may be marked as spam. Spam traps are older, inactive addresses repurposed by abuse monitors. Even a single send to one can trigger a sender reputation penalty.

Remove emails with invalid syntax (e.g., missing @ or TLD)
Filter out addresses ending in @admin, @support, @sales, @info
Eliminate addresses from disposable domains (e.g., @mailinator.com, @guerrillamail.com)
Exclude emails from catch-all domains (common in free or hosted providers)
Block known spam traps or outdated aliases (often identified through reputation tracking)

Use MailTester’s bulk verification tool to scan entire lists before syncing with Customer.io or Braze. It flags risks like catch-all domains and role-based addresses with 98.9% accuracy. Clean data isn’t optional—it’s the foundation of consistent deliverability.

How do you verify an email list before importing into Customer.io or Braze?

You verify an email list by testing each address in real time for syntax, domain validity, and SMTP responsiveness before importing into Customer.io or Braze. This stops bounces, protects sender reputation, and improves inbox placement. Use a real-time API during sign-up, and run bulk checks on existing lists to catch invalid, risky, or disposable emails early.

Start with real-time verification during data collection

Let’s be honest: collecting emails during sign-up is the cleanest moment to ensure quality. Use a real-time verification API to validate each address the second it’s entered. This stops typos, disposable domains, and non-existent accounts before they hit your CRM or email platform.

Many systems like Customer.io and Braze accept new contacts in real time. That means you can block invalid entries at the edge. With MailTester’s real-time verification API, you can integrate validation into your form workflow without slowing down conversions.

Run thorough bulk checks on existing lists

Existing lists often degrade over time. Domains expire. Users change jobs. Email providers block certain patterns. To fix older data, run a full bulk verification. This checks every address against SMTP, MX, syntax, and domain record validity—all in real time.

MailTester processes lists at scale with a 98.9% accuracy rate. It flags addresses that are syntactically wrong, don’t resolve to a valid mail server, or belong to catch-all domains—common culprits for high bounce rates. Removing these before importing into Braze or Customer.io significantly reduces delivery failures.

Test individual addresses during sign-up—use a real-time API to validate each email as it’s entered. This prevents invalid data from ever entering your system.
Validate domains and MX records—an email can pass syntax checks but fail if the domain has no MX record or if the server rejects connections.
Check for SMTP responsiveness—not all domains accept incoming mail. A true SMTP check simulates sending and waits for a response to confirm the mailbox is active.
Identify role accounts and disposable domains—addresses like admin@ or user123@ are often ignored or marked as spam. Disposable domains (e.g., mailinator.com) are non-functional long-term.
Use deliverability testing—even if an address is technically valid, it might end up in spam. Run inbox placement tests with real provider inboxes to verify deliverability before sending.

The 5 steps described in “Run thorough bulk checks on existing lists”, in order.

The goal isn’t perfection—it’s removing the known bad. You don’t need to catch every edge case, but you must eliminate the predictable ones. This reduces blocklist risk, improves sender reputation, and keeps more messages in inboxes.

For context, RFC 5321 and RFC 5322 define the technical standards for email transmission and syntax—validating against these standards is the foundation of robust verification. Tools that skip SMTP checks or rely solely on syntax rules miss critical delivery risks.

Try a test with your most recent list using MailTester’s bulk email checker. See which entries are risky before they cost you deliverability.

What does MailTester’s email-verification process actually check for?

You’re not just checking if an email exists—you’re validating the full path from syntax to inbox placement. MailTester runs a multi-step technical inspection: it verifies formatting, confirms the domain resolves, tests SMTP reachability, checks for catch-all domains, filters disposable addresses, and detects role-based accounts. These checks target real delivery risks before you send.

How each check reduces delivery risk

Syntax validation rules out malformed addresses (like [email protected] or user@@example.com), which fail at the earliest stage of delivery. A valid format is the first requirement for any email to even be processed.
Domain existence confirms the domain resolves via DNS. Without a valid domain, no mail server can receive messages. This step blocks typos and non-existent domains before waste occurs.
Mail server reachability tests whether the receiving mail server accepts connections. If the server doesn’t respond or rejects the connection, there’s no point sending. This avoids permanent bounces and protects sender reputation.
Catch-all detection identifies domains that accept all incoming mail, even for non-existent users. These are high-risk for engagement—they signal to inbox providers that the list may include fake or inactive addresses.
Disposable domain filtering flags known temporary email providers (like Mailinator, Guerrilla Mail). These domains are used for account creation and rarely open or engage with content.
Role account detection identifies common role-based addresses (sales@, info@, admin@). These have low engagement and can inflate spam complaints if used at scale.

Why this matters for Customer.io and Braze journeys

Automated journeys in Customer.io and Braze rely on clean data. If your list contains invalid or risky addresses, you hit higher bounce rates, which hurt sender reputation. ISPs like Gmail and Outlook monitor bounce trends closely. High bounce rates lead to filtering or blocklisting.

This is why many senders use tools like MailTester to pre-clean lists before launching campaigns. It’s not about guessing—email deliverability is a technical problem, and fixing it starts with knowing what’s actually valid and safe to send to.

For example, RFC 5321 specifies the SMTP protocol behavior for message acceptance. MailTester follows this standard in its server reachability checks. Similarly, Spamhaus maintains lists of known disposable domains, which MailTester cross-references in real time.

Want to test how your list performs in real inboxes? Try our inbox placement testing. For bulk validation, start with our bulk verification tool.

How can you test deliverability for real automated journeys before full send?

You can test deliverability for real automated journeys before sending to your full list by using inbox-placement testing that simulates actual delivery conditions across major inboxes like Gmail, Outlook, and Yahoo. MailTester’s inbox-placement tester sends real test messages through actual delivery paths, revealing whether messages land in the inbox, spam, or are blocked—plus exposes routing issues, filter logic, or authentication mismatches that would otherwise go unnoticed.

Testing real automated journeys with real inbox paths

Let’s say you’re setting up a welcome series in Customer.io or Braze. You don’t want to send it to 10,000 users only to find half are hitting spam folders. That’s why MailTester’s inbox-placement testing runs your message through real, live routes used by major email providers. It doesn’t rely on simulated or cached data—it uses actual infrastructure to mimic how your message would be received across Gmail’s filters, Outlook’s engagement scoring, or Yahoo’s reputation checks.

Each test sends a real message from your domain to real recipient accounts hosted at those providers. This means you’re not guessing—your results reflect actual inbox placement outcomes. You can test your message content, sender authentication (SPF, DKIM, DMARC), and headers in the environment they’ll face when live, identifying problems early.

What you learn from inbox placement results

After each test, you’ll see whether your message lands in the inbox, marked as spam, or is blocked entirely. More importantly, you’ll spot the root cause. For example, a mismatch in SPF or DKIM alignment can trigger spam filtering in Gmail even if everything else is correct. Similarly, poor sender reputation or a new IP address with no history may push your message to bulk folders in Outlook.

These insights come from testing across 15+ major inboxes using real delivery paths. The results help you fix issues before sending to your full audience. This approach is standard practice in email deliverability—RFC 5321 and RFC 5322 govern SMTP transport and message headers, which real inbox testers validate end-to-end. For a real-world example of how major senders use this, Spamhaus tracks reputation-based filtering at scale.

Use inbox-placement testing to validate your automated journeys from Customer.io or Braze before you send them. Test your setup, fix routing or authentication gaps, and confirm your messages will land where they should—before the first user sees them.

What real-world deliverability benchmarks should you aim for in Customer.io and Braze journeys?

You should aim for a delivery rate above 90% (excellent at 95%), a bounce rate under 1%, spam complaints below 0.1% (0.01% is excellent), and inbox placement exceeding 85% for non-promotional automated content. These targets reflect industry-standard thresholds for healthy sender reputation and sustained inbox access. Even slight deviations can impact long-term engagement and platform trust.

Delivery and bounce thresholds: your baseline health check

A delivery rate above 90% means your messages are reaching inbound servers without being rejected—this is solid performance.
Target 95% or higher for automated journeys; consistent 95%+ delivery signals strong list hygiene and sender reputation health.
Bounce rates above 2% usually mean your list has expired, invalid, or spam-trap addresses. This triggers scrutiny from anti-spam systems.
Aim for under 1% bounce rate; consistently over 1% suggests problems with data acquisition, validation, or outdated records.

Inbox placement and spam compliance: the ultimate test

For non-promotional automated journeys (e.g., welcome sequences, onboarding, account updates), inbox placement should exceed 85%.
Below 80% often means your messages are being flagged as low engagement or spam by filters. Check content, sending frequency, and subscriber intent.
Spam complaint rates should stay under 0.1%—any higher risks blacklisting or reduced email privileges from platforms like Gmail and Apple Mail.
Reaching 0.01% or below indicates excellent content quality, timing, and user consent management.

These targets are consistent with guidelines from industry watchdogs like Spamhaus and email ecosystem reports published by Return Path and Radicati. They’re not aspirational—they’re baseline expectations for sustained, trusted delivery.

Let’s be clear: no tool guarantees inbox placement. But consistently hitting these benchmarks across Customer.io and Braze journeys reduces the risk of being filtered or blocked. Use real-time verification before each send to catch issues early. Test your emails with a real inbox placement check before launching campaigns.

Regularly clean your lists. Validate new entries with verified address checks before adding them. For large-scale campaigns, use the bulk email list verification tool to detect invalid or risky addresses before sending.

How does MailTester’s 98.9% accuracy impact your automated journey success?

MailTester’s 98.9% accuracy means fewer wasted sends, no false flags on real customers, and fewer blocked messages. You’re not losing revenue to invalid addresses or risking sender reputation with bad data — just cleaner, more predictable automated journeys in Customer.io and Braze. That’s because we catch real problems—like typos and disposable domains—without tossing out valid users or missing known bad addresses.

Less noise, more signal: real accuracy means fewer surprises

False negatives — valid addresses marked invalid — mean you’re blocking real customers from receiving important messages. True, even small drops in list health can erode engagement. Conversely, false positives — invalid addresses marked valid — waste sends, hurt sender reputation, and can land you on blocklists. With MailTester’s precision, you don’t have to guess. You know, before you send, that your list is clean and your data is reliable.

Let’s be clear: most tools give you a guess. MailTester gives you a signal. Our accuracy comes from layered checks: SMTP validation, MX record lookups, disposable domain detection, and role account identification — all running in real time. This means your journey data is both accurate and actionable.

Deliverability and reputation don’t lie — and neither should your data

Every hard bounce or undeliverable message tells email providers you’re sending to bad data. That damages sender reputation over time, even if the message itself is legitimate. With lower bounce rates from clean data, you maintain stronger inbox placement. That’s not just good for volume — it’s essential for consistency, especially when scaling automated journeys.

It’s industry-standard that sender reputation is built on sending behavior, not just content. Tools like Mail-Tester (not to be confused with us) test inbox placement, while Spamhaus tracks blocklist activity — both underscore the value of clean sender data. When you prevent send failures at the source, you're not just improving deliverability today; you’re building long-term trust with platforms like Customer.io and Braze.

With MailTester, you’re not just verifying emails — you’re reinforcing the foundation of automated journeys. Whether you’re using our real-time verification API or running bulk checks via our bulk verification tool, every validation improves your next send. No more surprises. Just better outcomes.

What integrations make MailTester work seamlessly with Customer.io and Braze?

You can integrate MailTester directly into Customer.io and Braze using native APIs, so you verify emails before and after sending. Pre-verify addresses in your data pipeline to catch invalid, disposable, or risky emails early. After campaigns launch, schedule post-verification checks to maintain list quality over time. This reduces bounces, protects sender reputation, and improves inbox placement — all essential for automated journeys.

Pre-verification: Stop bad emails before they enter the platform

Use MailTester’s real-time verification API to validate email addresses during data ingestion, before they reach Customer.io or Braze.
Block catch-all, role-based, or disposable addresses before they enter your automated workflows — this prevents delivery failures and protects your sender reputation.
Integrate the API into your data pipeline using standard HTTP requests. It returns a verified status in under 1 second, so delays are minimal.

Post-verification: Keep your list healthy after deployment

Schedule periodic bulk verification jobs after campaign launches to check for changes in email validity over time.
Reputation and deliverability degrade when you send to stale or invalid addresses. Regular checks detect changes like domain deactivations or account deletions.
Use the native integrations with Customer.io and Braze to automate this process without manual export/import.

Industry standards like RFC 7830 define how email routing and delivery work — and they assume clean data. Sending to invalid or non-existent addresses wastes resources and harms long-term deliverability. Tools like MailTester help you meet those standards without adding complexity.

Even if you're using a platform like Braze or Customer.io with built-in filtering, those systems don't catch all invalid emails. You still need to verify at the source. MailTester fills that gap by doing more than just syntax checks — it validates real deliverability conditions.

Can you improve deliverability while keeping your email velocity?

You can improve deliverability without slowing down your email velocity by cleaning your list before sending. Low bounce rates and minimal spam complaints signal trust to platforms like Customer.io and Braze, helping maintain high inbox placement. With proper list hygiene, you avoid throttling and keep sending at scale.

Pre-send list cleaning is the foundation of high velocity

High-volume automated journeys rely on consistent sending patterns. If your list contains invalid or risky addresses, your sender reputation suffers — even if you’re sending at a steady pace. Sending to bounce-prone addresses triggers rate limits or blocks, especially when platforms like Customer.io or Braze detect anomalies. Cleaning your list upfront prevents these issues and keeps your velocity stable.

Tools like MailTester’s bulk verification identify invalid, caught-all, or disposable addresses before they get sent. You’re not reducing volume — you’re optimizing it. This means fewer bounces, fewer spam reports, and better delivery results on every send.

Reputation is built on consistency, not speed

Platforms like Customer.io and Braze evaluate sender reputation over time based on engagement, bounce rate, and spam feedback. A sudden spike in bounces or a poor sender reputation history can lead to throttling — even if your sending volume is low. You can’t outpace poor list quality.

By maintaining low bounce and spam rates, you signal to these platforms that your content is trusted. This is a long-term benefit: consistent hygiene improves deliverability across all automated journeys, not just one campaign. It’s not about sending more — it’s about sending smarter.

For real-time integration, MailTester’s email verification API allows you to validate addresses at signup or during journey triggers. This keeps your list clean from the moment it’s created. And since MailTester’s accuracy is over 98.9%, you can trust the results without overfiltering legitimate users.

The goal isn’t to slow down — it’s to send sustainably. With clean data, your automated journeys run faster because deliverability doesn’t degrade over time. You’re not just keeping velocity; you’re improving it through reliability.

How does MailTester help you avoid sending to disposable domains in automated flows?

Disposable email addresses are common in automated journeys, especially during onboarding or campaign sign-ups. They often lead to high bounce rates, poor engagement, and can harm your sender reputation.

MailTester flags disposable domains in real time using a curated database of known temporary email providers. This detection happens during verification, so you know which addresses are risky before any message is sent.

By blocking these domains at the verification stage, you prevent delivery to temporary addresses. This reduces wasted sends, improves inbox placement, and helps maintain a clean sender reputation—without manual filtering or guesswork.

Sources

Sending from a domain with at least three months of history improves inbox placement by 28% compared with a brand-new domain. — Woodpecker data (via WarmForge deliverability statistics) (2025)
The platform-wide average cold email reply rate is 3.43%, while the top 25% of senders achieve 5.5%+ and the top 10% reach 10.7%+, based on billions of emails sent in 2025. — Instantly Cold Email Benchmark Report 2026 (via Satellyte) (2026)

Keep reading

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Frequently asked questions

What happens to an email if it’s flagged as 'catch-all' during verification?

It’s marked risky. Sending to catch-all domains increases spam likelihood — many inboxes treat these as disposable or low-value. Such addresses should be excluded from automated journeys.

Can I verify emails in bulk for a Customer.io journey?

Yes. MailTester’s bulk verification API processes thousands of addresses at once, returning detailed results including verdicts (valid, invalid, catch-all, risky) for each.

How often should I clean my list for Braze automated campaigns?

At least quarterly, and always before launching a major campaign. Also verify new sign-ups in real time using the API.

Why do spam traps hurt deliverability even if they’re not active?

If you send to a spam trap — even accidentally — it signals poor list hygiene to inbox providers. They may reduce trust in your sender domain and increase filtering of future emails.

Does verifying emails improve sender reputation?

Yes. Reducing bounces, spam traps, and invalid addresses improves sender reputation over time, which directly supports inbox placement for automated messaging.

Is MailTester’s real-time verification API fast enough for high-volume sign-ups?

Yes. It provides sub-second response times and scales to thousands of verifications per minute, suitable for real-time sign-up flows in Customer.io and Braze.

Do unused or old addresses degrade deliverability?

Yes. Inactive or outdated addresses often belong to role accounts or disposable domains. They can become spam traps or generate bounces, triggering filters.

Can I test my automated journey setup before sending?

Yes. MailTester’s inbox-placement tests simulate real delivery paths to major inboxes, showing expected results before full send.

Does MailTester work with all email providers used by customers?

It checks address validity across all major domains, including Gmail, Outlook, Yahoo, and corporate email systems like those in Salesforce or Microsoft 365.

What if my list includes role-based addresses like 'marketing@'?

These are high-risk and often bounce. They should be filtered out before automation unless you have explicit opt-in and engagement data.

Can I use MailTester with SendGrid, Klaviyo, or other platforms?

Yes. MailTester integrates with Mailchimp, HubSpot, Klaviyo, SendGrid, and other platforms to verify before delivery, across all automation workflows.

What happens if I don’t verify my email list?

High bounce rates, increased spam scores, and sender reputation damage — all leading to poor inbox placement even for well-designed automated journeys.