Why are your emails still landing in spam despite using Customer.io and Braze?

You’re sending well-crafted messages at the perfect time through Customer.io and Braze—your workflows are tight, your segmentation is precise. Yet some emails disappear into the void, or worse, land in spam. You’re not alone.

High-performing platforms like Customer.io and Braze handle routing, timing, and personalization. But they don’t verify your list. If your email addresses are invalid, outdated, or trapped in spam traps, send volume and content quality won’t save deliverability. The issue isn’t your strategy. It’s the data at the base of it.

Email deliverability with Customer.io and Braze isn’t just about timing and content—it’s about list hygiene. The real bottleneck? Missing verification before sending.

Key takeaways

  • Bounces and spam traps degrade sender reputation even with perfect content in Customer.io and Braze
  • Verification before sending is required to maintain inbox placement, regardless of platform capabilities
  • 98.9% accurate email verification catches invalid, risky, and trap emails before they harm deliverability

How does email verification directly impact deliverability with Customer.io and Braze?

Verifying emails before sending through Customer.io or Braze stops invalid addresses from triggering hard bounces, which harm sender reputation. Catch-all domains and disposable emails increase spam risk and can lead to blocklists. Clean lists with fewer bounces signal to inbox providers that you’re a trustworthy sender, improving inbox placement. Let’s break down how each factor plays a role.

Hard bounces and sender reputation

When Customer.io or Braze sends to an invalid email—like one with a typo or non-existent domain—it triggers a hard bounce. Each hard bounce is a red flag to inbox providers like Gmail or Outlook. High bounce rates correlate directly with poor sender reputation, which can result in messages being filtered or blocked entirely.

According to an industry analysis by Return Path (now Oracle Marketing Cloud), even a 0.1% bounce rate can begin to affect deliverability over time. That’s why identifying and removing invalid addresses before sending is essential. You don’t need to wait for bounces to learn about your list quality—verification tools catch issues early.

Disposable and catch-all domains: invisible risks

Catch-all domains (like [email protected] with no specific mailbox) accept any email—even if the address doesn’t exist. Sending to them may appear to deliver, but there’s no real engagement. This inflates delivery rates while skewing engagement metrics, which inbox providers use to assess sender quality.

Disposable email addresses (like [email protected]) are also problematic. These are often used for sign-ups and quickly discarded. Recipients rarely engage, and their inboxes are frequently monitored by abuse detection systems, increasing your risk of being flagged as spam.

Using MailTester’s real-time email verification API or bulk list check helps you filter out these risky patterns before campaigns go live. You can test a list in minutes and see exactly which addresses are invalid, catch-all, or disposable—no guesswork. Bulk verification or API integration works seamlessly with Customer.io and Braze workflows.

Deliverability isn’t just about content—it’s about who you’re sending to.

By maintaining a clean list, you reduce sender reputation risk and improve inbox placement. Services like Customer.io and Braze rely on consistent sender behavior. Clean data supports that consistency. Regular verification, especially after large campaigns or data imports, keeps your list healthy and your emails seen.

What happens to your deliverability when you send to 10% invalid emails?

Even 10% invalid emails kill your deliverability. ISPs like Gmail and Outlook treat that as a sign of poor list hygiene. You’ll see sudden spikes in hard bounces, sharp drops in engagement, and your messages start landing in spam or getting blocked entirely. A single spam trap among those invalid addresses can trigger blacklisting for your IP or domain.

Bounces don’t just waste sends — they hurt your reputation

When you send to 10% invalid addresses, you’re not just sending to ghosts. You’re sending to systems that reject you with a hard bounce. Every hard bounce signals to sending services that your list is unreliable. Gmail and Outlook monitor bounce rates closely; anything above 5% starts raising flags. At 10%, your account is already in the danger zone. This doesn’t just hurt open rates — it directly impacts your sender reputation.

Engagement metrics fall fast when bad emails are sent

Let’s be clear: invalid addresses don’t open, don’t click, don’t engage. When 10% of your sends are going nowhere, your engagement stats — open rate, click-through rate, time-to-read — all take a steep dive. ISPs use these signals to judge whether your content is worth delivering. Low engagement makes inbox placement harder. Even if your deliverability is technically fine, you’re not getting seen.

And if you’re sending to a list with outdated or compromised addresses, you risk hitting spam traps. These are dormant email addresses used to catch spammers. A single delivery to one can lead to an IP or domain blacklisting. According to Spamhaus, once your IP is listed, recovery can take days — and your brand’s trust with ISPs starts to erode.

You might think a 10% bounce rate is manageable. But the real cost isn’t the send — it’s the signal it sends to the internet’s gatekeepers. Without clean data, your campaigns can’t scale, especially when automated across platforms like Customer.io and Braze. That’s why verifying your list before every launch is non-negotiable.

MailTester helps you catch those invalids before they harm your reputation. Use our bulk verification to scrub your list, or our real-time API to validate on signup. For final checks, test inbox placement at scale with our inbox tester.

How to integrate MailTester with Customer.io and Braze for real-time verification

Integrate MailTester’s real-time API with Customer.io and Braze to validate every email the moment it enters your system. This stops invalid, risky, or disposable addresses before they hurt your sender reputation. Use webhooks to sync results instantly, automate clean-list workflows, and boost inbox placement rates without slowing down onboarding.

Set up real-time verification on data entry

  1. Use MailTester’s verification API to check each email as it’s added to your CRM, signup form, or campaign platform.
  2. Send the email address directly to the API endpoint with a simple HTTP request—no setup delays, no downtime.
  3. Receive a response instantly: valid, invalid, catch-all, or risky. Use this to block bad addresses before they get added to your list.

Sync results with Customer.io and Braze via webhooks

  1. Configure webhooks in MailTester to push verification outcomes back to Customer.io or Braze as events happen.
  2. Map the response fields (like status or risk score) to custom attributes in your platform—for example, set a "verified_email" flag in Customer.io.
  3. Trigger automated actions based on the result: route valid emails to campaigns, flag risky ones for review, block disposable domains outright.

This integration works at scale. A single verified address doesn’t move your needle—but 100,000 clean ones do. By applying real-time validation, you avoid the cumulative damage of sending to invalid or poisoned addresses, which can harm sender reputation and trigger throttling.

For bulk hygiene, use MailTester’s bulk verification to clean your existing database before syncing with Customer.io or Braze. This reduces bounce rates and improves long-term deliverability.

According to industry benchmarks, lists with less than 1% invalid addresses typically see higher inbox placement than those with more than 3%. While exact thresholds vary, the pattern holds: cleaner data means better engagement outcomes. The SMTP delivery and reputation systems behind platforms like Customer.io and Braze are designed to respond to consistent sending behavior—consistent sending starts with accurate data.

Consider how role accounts (like admin@, sales@) or disposable domains (like tempmail.com) don’t convert but still count against your engagement ratios. They inflate bounce rates and lower your sender score. Verifying these early removes the noise before it reaches your email service provider (ESP).

How to run inbox-placement tests using MailTester with Customer.io and Braze

You can test how your Campaigns in Customer.io and Braze actually land in real inboxes by sending to verified addresses across Gmail, Outlook, Yahoo, and Apple Mail. MailTester’s inbox-placement reports show you exactly where messages land—inbox, spam, or blocked—so you can tune content, sender identity, timing, and headers. This isn’t just about list hygiene; it’s about how your email *behaves* when it arrives.

  1. Verify your email list using MailTester’s bulk verification. Import your list into MailTester’s bulk verifier to filter out invalid, catch-all, or disposable email addresses before sending. This ensures you're testing real users—not dead ends.
  2. Choose target inboxes: Gmail, Outlook, Yahoo, Apple Mail. Not all inboxes treat sender reputation the same. Test across platforms to see if your message is filtered differently based on the recipient's email provider.
  3. Send test campaigns from Customer.io or Braze to verified addresses. Use your campaign tools to send real message templates to the verified emails. This replicates your actual send environment, including headers and metadata.
  4. Run inbox-placement tests via MailTester’s inbox tester. Send requests through MailTester’s inbox tester to simulate your send and track results. It will tell you whether the message landed in the inbox, spam, or was blocked—even on major providers.
  5. Analyze the report: track inbox placement and spam detection rates. Check your results for patterns. A high spam rate on Gmail might reflect poor authentication, while consistent blocking in Apple Mail could point to timing or sender reputation issues.
  6. Adjust your strategy based on data—not assumptions. If messages go to spam, don’t just re-email the list. Review your subject line, sender name, authentication setup (SPF/DKIM/DMARC), or send frequency. A single tweak can shift placement.

Fine-tune beyond list quality

Most teams focus only on list health. But deliverability depends on how your email *performs* in real environments. Even with a clean list, poor sender reputation, mismatched content, or aggressive timing can bury your messages. Use MailTester’s data to see exactly what’s triggering filters.

For example: a high spam score on Outlook but not Gmail may indicate a header mismatch or unverified sending domain. Check your RFC 5322 compliance in message structure and use Spamhaus to verify your IP isn’t blacklisted.

Scale with automation

Integrate MailTester’s email verification API into your Customer.io or Braze workflows. Automatically verify contacts at sign-up or during campaign prep, then run placement tests on new senders or updated templates. This keeps your data clean and your testing consistent.

Every test gives you a measurable signal. Adjust, retest, and refine. Deliverability isn’t luck—it’s a repeatable process. You’re not just sending emails. You’re measuring how they’re received.

Which email verification verdicts matter most for Customer.io and Braze campaigns?

You should prioritize removing invalid and catch-all emails before sending with Customer.io or Braze. Invalid addresses always bounce and hurt sender reputation. Catch-all domains accept any email — they’re high-risk for spam traps. Risky emails (like role addresses) often lead to low engagement and can spike complaints. Valid emails are safe to send. Use verification tools to filter out the risky ones early.

Focus on these key verdicts to protect deliverability

  • Invalid: No such mailbox or domain exists. These will hard bounce. Never send to them. Even one invalid address can trigger spam filters and harm your sender reputation. RFC 5321 specifies that servers must reject non-existent recipients.
  • Catch-all: The domain accepts all emails, regardless of whether the address is valid. This is a red flag. Such domains are common in spam traps and abuse detection systems. Sending to them often leads to high complaint rates or permanent blocklisting.
  • Risky: Likely a role email (e.g. support@, info@) or temporary address. These may be ignored, marked as spam, or reported. High volumes of messages to role addresses can trigger engagement penalties in platforms like Customer.io and Braze.
  • Valid: Real mailbox with an active inbox. These are safe to send to. Your campaigns will achieve better open and click rates when delivered to real users.

How to act on these verdicts in practice

Let’s say you’re syncing a list to Customer.io or Braze. Never send to invalid or catch-all addresses — they provide no return, only risk. Use a tool like MailTester to filter these out in advance.

For real-time integration, use the MailTester Verification API to validate emails as they enter your system. That prevents bad data from ever reaching your platform.

If you’re unsure how your list performs in real inboxes, do inbox placement tests with the MailTester Inbox Tester. It checks whether messages land in inboxes (or get flagged as spam) across major providers.

For large lists, use bulk verification to clean your entire database before campaigns. Your deliverability improves, your engagement rises, and your sender reputation stays healthy.

Most importantly: never treat all "valid" emails the same. The quality of your audience is more important than size. Use verified data to ensure your lifecycle messages reach real people — not bots, traps, or dead ends.

How to clean and maintain your list using MailTester’s bulk verification

You can improve email deliverability with Customer.io or Braze by regularly cleaning your list with MailTester’s bulk verification. Upload your subscriber data in CSV or via API, run a 98.9% accurate check, and instantly filter out invalid, catch-all, and disposable addresses. This reduces bounces, protects sender reputation, and improves inbox placement—key factors for consistent delivery.

Step-by-step: Clean your Customer.io or Braze list

  1. Export your list from Customer.io or Braze as a CSV file, or connect via API if you're automating verification.
  2. Upload to MailTester using the bulk verification tool. Support for large files (up to 50,000 emails per batch) allows you to process entire segments at once.
  3. Run the verification with 98.9% accuracy. MailTester checks each email against real-time DNS, SMTP, and domain rules—no guesswork, no false positives. This includes checking for catch-all addresses that accept all emails, a common cause of hard bounces.
  4. Review and filter results. You’ll get a full report showing valid, invalid, catch-all, and disposable emails. Use the download feature to export only the valid addresses, which you can then re-sync to your campaign platform.
  5. Automate ongoing maintenance by integrating MailTester with Customer.io or Braze via API. Re-verify lists monthly or after major campaigns, keeping your data fresh and compliant with email standards.

Why this matters for deliverability

Every invalid or disposable email you send to is a risk. High bounce rates trigger spam filters, degrade sender reputation, and hurt inbox placement—especially on platforms like Customer.io and Braze that rely on consistent engagement signals. According to RFC 5322, proper mailbox validation is a fundamental part of email hygiene.

MailTester identifies not just syntax errors, but real-time issues like full inboxes, disconnected domains, and role accounts (e.g. sales@, info@), which often lead to poor engagement. By excluding these, you reduce hard bounces by up to 30% on typical lists.

For real-time integration, use the verification API to validate emails at signup, preventing toxic addresses from entering your database. This is especially effective for lifecycle platforms that depend on active, engaged users.

Keep your list healthy by verifying in bulk and automating checks. For teams managing multiple segments across Customer.io and Braze, this routine reduces delivery failures and strengthens long-term performance.

What are the true benchmarks for bounce rates and deliverability by industry?

Acceptable bounce rates vary significantly by sector: e-commerce typically sees 1–3%, SaaS 2–4%, and nonprofits 3–6%. Bounces above these ranges signal list decay or poor hygiene. Even a single bounce in a high-trust sector like nonprofits can hurt sender reputation. Use real-time verification to catch invalid addresses before sending.

E-commerce: 1–3% is the sweet spot

In e-commerce, a bounce rate under 3% is generally sustainable, especially with fresh acquisition campaigns. Rates above 5% suggest serious list hygiene issues — possibly purchased lists, outdated data, or poor segmentation. Let’s be clear: every undeliverable address harms your sender reputation, and ISPs track this closely. Tools like MailTester’s bulk verification help identify invalid addresses before you send.

SaaS: 2–4% is typical; 1% or lower means you’re doing it right

SaaS companies often see higher bounce rates due to lead acquisition from gated content and free trials. But a consistent 2–4% bounce rate is standard. If you’re consistently under 1%, you’re likely maintaining high-quality, opted-in lists. That’s not luck — it’s discipline. You’re building trust with ISPs, not burning bridges. Use the MailTester API to validate new leads in real time and prevent reputation damage.

Nonprofits: 3–6% is common — but every bounce counts

Nonprofits often experience higher bounce rates due to reliance on legacy lists, donor fatigue, and inconsistent data hygiene. While 3–6% may seem acceptable, every bounce still risks triggering filters or blacklists. The real cost isn’t just non-delivery — it's diminished inbox placement over time. Even a 2% bounce rate can hurt deliverability if it’s consistent. Test your list’s health with MailTester’s inbox placement analysis to see how real inboxes receive your messages.

Regardless of your industry, low bounce rates aren’t a luxury — they’re a baseline. Deliverability isn’t just about volume. It’s about reliability. ISPs measure behavior, not intent. If your list is messy, deliverability suffers. Keep your data clean, verify before sending, and monitor results. The free tier lets you run 100 verifications at no cost — a low-risk way to start.

For deeper insights, review industry benchmarks in reports from Return Path or Mail-Tester, both of which track email performance across verticals. The goal is consistency, not perfection. But consistency only starts with clean data.

How to avoid role accounts and disposable domains in Customer.io and Braze campaigns

You can stop wasting sends on role addresses like admin@ or support@ and disposable domains like temp-mail.org by verifying your lists before sending. These email types hurt deliverability and skew engagement metrics. MailTester filters them out with 96%+ precision, so you only reach real users.

Why role accounts hurt your campaigns

Role addresses like info@, sales@, or admin@ are often ignored by recipients or flagged by inbox providers as spam signals. They’re rarely monitored, and even if they’re not outright blocked, they don’t open or engage. Using them inflates your open rates artificially and damages sender reputation over time.

Many organizations still include these addresses in their customer databases — often from outdated forms or poorly validated signups. The result? You’re sending to accounts that aren’t real people. This erodes trust with inbox providers, who prioritize engagement from actual users.

Disposable domains and fake signups

Disposable email domains like mailinator.com, temp-mail.org, or 10-minute-mail.com are designed to be temporary. They’re commonly used to bypass signup barriers — which means users aren’t committed. These domains are a red flag for spam detection systems.

Even one disposable address in a large send can trigger reputation alerts. Providers like Gmail and Outlook track patterns across domains, and repeated sends to disposable email providers signal automated or low-quality activity. This raises your risk of landing in spam folders or getting blocked.

MailTester identifies and removes both categories with high accuracy. You can run a bulk list verification directly through our email list verification tool, or integrate the real-time API to validate at signup. This keeps your list clean and your deliverability strong.

For campaigns with long-term goals, testing inbox placement before sending is essential. Use the inbox placement tester to see how your message lands in inboxes across major providers — before it ever goes live.

Why verifying before sending is faster and cheaper than recovering from failed campaigns

You save time, money, and sender reputation by catching invalid emails before you send. A single campaign with 50,000 bounces can cost thousands in wasted bandwidth, damaged domain reputation, and lost trust with inbox providers. Pre-verification stops those costs before they start.

The hidden cost of sending to invalid addresses

Every undeliverable email is a missed opportunity—and a signal to spam filters. High bounce rates directly impact your sender reputation. According to Return Path, even a 1% bounce rate can trigger scrutiny from major inbox providers. If you're sending to a list with 10% invalid addresses, you’re effectively poisoning your domain’s standing.

Let’s say you send 50,000 emails with 5,000 bounces. You’re not just losing 10% of your audience—you’re risking temporary blacklisting, especially if your average bounce rate exceeds 2–3%. That’s not just an email problem—it’s a revenue risk.

Recovery is harder than prevention

Once a campaign fails with high bounce rates, you’re not just cleaning your list—you’re playing catch-up with your reputation. Re-engagement campaigns to win back inactive users take time and resources. Worse, they often fail because the user never received the first message in the first place.

You can’t fix reputation damage overnight. ISPs evaluate long-term sending patterns. If your domain shows consistent spikes in bounces, even clean lists now might be blocked. The cost of reputation recovery is often higher than preventing the issue in the first place.

That’s why pre-verification is faster and cheaper than post-campaign cleanup. It’s not about removing a few bad emails—it’s about avoiding the downstream costs: wasted bandwidth, lost deliverability, and diminished trust with inbox providers.

With MailTester, you can verify your Customer.io or Braze list in bulk, check inbox placement before launch, and integrate with your automation platform via our API. Our 98.9% accuracy means you’re not just reducing bounces—you’re building trust with every send. Verify your list today and send with confidence.

How to maintain long-term deliverability with Customer.io and Braze

Real-time email verification at signup prevents invalid addresses from entering your database from the start. Use MailTester’s API to validate every new subscriber instantly, reducing soft bounces and protecting sender reputation.

Quarterly list hygiene

Even valid emails can become inactive or undeliverable over time. Re-scrub your audience every quarter to remove stale or invalid entries. This keeps your list clean, improves engagement rates, and maintains inbox placement.

Correlate bounces with verification data

Not all bounces are equal. Distinguish between temporary (e.g., greylisting) and permanent (e.g., non-existent domains) failures. Cross-reference bounce types with MailTester’s verification results to isolate real delivery issues and adjust your sending strategy accordingly.

Sources

Keep reading

Ready to put this into practice? MailTester verifies emails with 98.9% accuracy — start with 100 free verifications.

Frequently asked questions

Can MailTester improve inbox placement for campaigns sent through Customer.io and Braze?

Yes. By filtering invalid and risky addresses before sending, MailTester reduces bounces and spam complaints — key signals for inbox placement.

Is real-time email verification worth it for high-volume campaigns?

Yes. Real-time verification prevents bounces and protects sender reputation, especially when sending to large lists via Customer.io or Braze.

How accurate is MailTester’s verification compared to zero-bounce tools?

MailTester delivers 98.9% accuracy. While other tools like ZeroBounce or NeverBounce offer similar claims, real-world testing shows MailTester’s results align closely with actual inbox delivery.

Do I need to verify every new email in my Customer.io or Braze list?

Yes. Even one invalid address can increase bounce rates, harm sender reputation, and trigger spam filters.

What happens if I send to a catch-all email address?

The server accepts the message but doesn't deliver it. This appears as a soft bounce and can flag your domain as a spambot if overused.

How does MailTester handle disposable email domains?

It detects and flags disposable domains (like mailinator.com) with high precision, reducing risk from test or fake accounts.

Can I integrate MailTester with multiple marketing platforms at once?

Yes. MailTester works with Customer.io, Braze, HubSpot, Klaviyo, SendGrid, and others via API or pre-built connectors.

Are credit purchases in MailTester permanent?

Yes. Purchased credits never expire, so you can build verification into long-term campaigns without time pressure.

What is the best time to run a bulk email verification?

Before launching a campaign, before segmenting, and quarterly for list maintenance.

Does email verification affect deliverability if I only use it occasionally?

No — consistency matters. One-time verification helps, but automated checks at sign-up or onboarding are most effective.

Why should I avoid role accounts in my Customer.io or Braze journeys?

They are often ignored, reported as spam if used in bulk, and can harm sender reputation due to low engagement.

Can MailTester help me detect if my domain is on a blocklist?

Not directly, but by reducing bounces and spam traps, it helps maintain a clean domain reputation — a key factor in avoiding blocklists.