Detecting Malicious Email Routing

What happens when an email shows multiple received lines from unverified proxies?

You open an email that claims to be from your bank. It has a familiar subject line, but something feels off. You check the headers and see multiple Received lines—each from a different server, none of which you recognize. That’s not how legitimate email chains work.

When an email contains multiple Received lines pointing to unverified proxies, it usually means someone routed the message through unauthorized servers. This isn't accidental—it’s a common tactic in phishing and spoofing campaigns. Legitimate senders don’t route messages through untrusted relays without proper authentication. The presence of these paths isn’t just odd; it’s a red flag.

MailTester’s verification engine detects these anomalies by tracing the full SMTP path and validating the legitimacy of each hop. We don’t just check if an address exists—we analyze how it got there.

Key takeaways

Multiple Received lines from unverified proxies strongly suggest the email was routed through unauthorized servers, a hallmark of phishing and spoofing.
Legitimate senders use authenticated, direct delivery paths; arbitrary relay chains with unverified hops are a reliable indicator of malicious intent.
MailTester’s engine analyzes the complete SMTP path and source legitimacy to flag anomalies that simple syntax checks miss.

How do multiple received lines reveal malicious routing patterns?

Each 'Received:' header in an email’s trace records a step in its journey. If you see multiple hops from unrelated or unverified domains—especially those without valid IP addresses or DNS records—it’s a strong sign the message is being routed through compromised infrastructure, proxies, or relay systems designed to hide the real sender. This pattern is common in spam campaigns, phishing attempts, and abuse-of-service attacks where the goal is to evade detection.

The anatomy of a clean delivery path

Legitimate emails typically have a clear, sequential chain: your mail server → your ISP’s outbound gateway → the recipient’s mail server. Usually, this path involves two to four hops, with each step resolving to a known, verified domain and IP. These entries align with standards defined in RFC 5322, the foundational specification for email message format.

Red flags: multiple unrelated headers and proxy abuse

When a message shows multiple ‘Received:’ lines from domains that don’t resolve to public IPs, lack proper SPF/DKIM records, or are unrelated to the sender’s origin, it suggests proxy abuse. For example, a message from a corporate domain appearing to pass through a third-party mail relay in a country with no known infrastructure is a red flag. Many such systems are hosted on cloud providers but configured without proper authentication—making them easy to exploit.

Attackers use this technique to mask their identity, avoid blacklists, and bypass spam filters. The more disjointed the path, especially if it loops through domains with no reverse DNS, the more likely it is malicious. These signals are used by mail reputation services—like those from Spamhaus and Barracuda—to score and block high-risk traffic.

MailTester’s email verification process checks sender reputation and validates the authenticity of each hop in the trace. By detecting anomalies like invalid or missing DNS records in the header chain, MailTester helps prevent messages from being routed through compromised gateways. Use our email checker to validate a single address before sending, or bulk verify a list to uncover hidden risks in your sender list.

Why unverified proxies in Received lines are a security risk

When an email shows multiple Received lines from unverified proxies, it means the message passed through suspicious or untrusted servers. These proxies often lack proper authentication—SPF, DKIM, and DMARC fail to align—which makes the email look forged or malicious. Attackers exploit this to hide their real origin while sending spam at scale, making detection and blocking harder. You can’t trust emails with a chain of unverified hops.

How unverified proxies compromise email integrity

Unverified proxies might be hijacked mail servers, open relays, or botnet-controlled nodes. These systems don't enforce proper sender authentication, so an email claiming to come from your company might actually be routed through a misconfigured or compromised server. The absence of valid SPF, DKIM, and DMARC alignment means the email fails alignment checks, which security filters increasingly flag as a red flag.

You’ve seen this in practice when bulk spam arrives with multiple Received headers from unfamiliar domains or IP ranges. Each hop adds a layer of obfuscation, making it harder to trace the original sender. This is especially common in phishing campaigns and malware distribution, where attackers prioritize stealth over delivery speed.

According to the RFC 5322, Received lines should reflect legitimate mail server handoffs—not a chain of unauthenticated intermediaries. When you see multiple lines from unknown or unverified sources, the message is statistically more likely to be malicious. The longer the path, the higher the risk.

Why attackers use multiple proxies to mask their tracks

Attackers use multiple unverified proxies to split their outbound traffic across dozens of IP addresses and domains, evading blacklists and rate-based detection. A single compromised server might send a few hundred emails before getting flagged—but a distributed network of unverified proxies can send tens of thousands without triggering alarms.

These proxy chains often show up in headers as multiple Received lines from domains not owned by your organization. You don’t control these servers. They aren’t authenticated. They’re essentially blind spots in your email security posture.

Let’s be clear: a clean inbox placement test isn’t enough if your inbound email stream includes messages with unverified Received lines. You need to audit the header chain for suspicious hops. Tools like MailTester’s inbox placement tester help assess deliverability and identify routing anomalies before they hit your users.

How MailTester detects malicious routing via Received line anomalies

MailTester scans every Received line in an email header to trace the path from sender to recipient, flagging any hop that routes through unverified proxies, greylisted domains, or known spam infrastructure. If a Received line leads to a domain with no MX record, a missing SPF record, or a non-routable IP, it's marked as high-risk. These anomalies often signal malicious rerouting, and MailTester uses this data to assign a 'risky' or 'invalid' verdict.

How the detection process works

Full header parsing during real-time verification. MailTester processes complete email headers—including every Received line—to reconstruct the full delivery path. This includes identifying each server and timestamp in the chain, which allows us to spot suspicious hops.
Cross-referencing each hop against threat intelligence. Every server in the chain is checked against known proxy lists, greylisted domains, and blacklisted IPs using real-time databases. If a hop matches a known proxy or suspicious network block, it triggers a red flag.
Validating DNS and SPF records at each stage. For any domain in the Received line chain, MailTester checks for a valid MX record, a correctly configured SPF record, and IPv4/IPv6 routing reachability. A missing MX or SPF, or a non-routable IP, indicates an unstable or fake endpoint.
Applying pattern recognition to assess risk. The system analyzes anomalies like rapid hops between unrelated domains, multiple Received lines from the same proxy IP, or unlisted third-party relays. These patterns are commonly seen in phishing, credential harvesting, and spam campaigns.
Assigning verdicts based on accumulated risk. If multiple red flags appear—such as unverified proxy hops paired with missing SPF or invalid MX records—MailTester labels the address as 'risky' or 'invalid' with clear rationale.

Why this matters for sender reputation and deliverability

Malicious routing often hides behind forged headers or compromised services. If your list includes addresses routed through unverified proxies, your IP is at risk of being flagged by providers like Gmail or Outlook. These systems track sender path integrity, not just content.

The 5 steps described in “How the detection process works”, in order.

According to the RFC 5321, the Received: header chain should reflect a legitimate path. When it doesn’t, the message fails basic SMTP integrity checks. MailTester identifies deviations from this standard before they damage your sender reputation.

Use MailTester’s quick email checker to review individual addresses, or verify entire lists in bulk to catch routing anomalies across thousands of addresses. The system runs these checks on every send, so you know your list is clean before deployment.

For teams using SendGrid, HubSpot, Klaviyo, or Mailchimp, integration is seamless—verifying email paths becomes part of your workflow without added complexity.

What a 'risky' verdict means in the context of multiple proxy hops

A 'risky' verdict indicates the email address likely routes through one or more unverified proxy servers—often signifying compromised accounts, disposable services, or misconfigured forwarding platforms. These routing patterns show up in multiple Received lines during SMTP handshake analysis, which can flag the address as high-risk for deliverability, spam filtering, or abuse. You’re better off skipping these addresses to protect your sender reputation.

Why multiple Received lines raise red flags

When an email passes through multiple proxy hops, each hop adds its own Received header. Legitimate email chains usually show one or two clear, trusted Received lines from verified domains. Multiple, inconsistent, or nested Received lines from unknown or disposable domains suggest the address isn’t tied to a genuine, direct endpoint.

According to the Internet Message Format (RFC 5322), Received headers should reflect the actual path mail took through trusted infrastructure. A heavily layered path with no clear origin point—especially with proxies not listed in public DNS records—can indicate abuse, data harvesting, or even compromised devices.

What 'risky' really means for your sends

A 'risky' address isn't definitively invalid, but it’s statistically more likely to result in hard bounces, spam traps, or failure to land in the inbox. MailTester detects these patterns using real-time SMTP checks and DNS analysis, including verification of MX records, TLS configuration, and the integrity of mail flow traces.

Accounts routed through untrusted proxies often belong to disposable email services (like temporary mailbox generators) or are hijacked. These are commonly used in credential stuffing, phishing campaigns, or botnet activity. Sending to them wastes bandwidth, increases spam risk, and can indirectly harm your sender reputation over time.

You can test how such addresses perform in real inboxes using our inbox placement tester. This shows whether your message lands in the primary inbox, spam folder, or fails entirely—helping you spot risky deliveries before they harm your reputation.

Even one risky address in a large list can trigger filtering, especially if your sending volume is high. Regular list hygiene—using tools like the bulk email verification—helps maintain a clean sender profile and ensures only verified, trusted addresses receive your messages.

The role of DNS and MX in validating received line legitimacy

You can detect malicious email routing by verifying that each Received line refers to a domain with an authoritative MX record pointing to a publicly reachable IP address. If a domain lacks an MX record or resolves to an IP without proper reverse DNS, it’s an unverified proxy — a red flag indicating potential abuse. MailTester checks every hop in the path using these standards.

How DNS records confirm legitimacy at each hop

Each Received line in an email header lists a server that handled the message. For that server to be trustworthy, its domain must resolve via DNS to a legitimate mail server. MailTester checks the MX record of every domain mentioned in the Received lines. If no MX exists, or the associated A/AAAA record doesn’t resolve to an open, publicly accessible IP, the server is flagged as suspicious.

Even if a domain has an MX record, we go further. We verify that the IP address has a correctly configured reverse DNS (PTR) record matching the domain. This is part of a broader practice that ensures the sender’s identity is not spoofed. According to RFC 5321, this alignment is a baseline for trusted mail flow. Tools like MxToolbox and Spamhaus often flag IPs without proper PTR as high-risk, which supports this validation step.

What unverified proxies reveal about sender behavior

Domains with no MX record, unresolvable IPs, or missing PTR records are common in spam and phishing campaigns. Attackers often route messages through compromised servers or cloud endpoints with no public DNS footprint. These are unverified proxies — they appear in Received lines but can’t be validated as genuine mail servers.

MailTester automatically flags these domains as unverified during bulk and real-time verification. This helps you avoid sending to addresses that are either invalid or tied to malicious infrastructure. The check isn’t speculative — it’s based on real-time DNS lookups across authoritative servers, including those that support DNSSEC for tamper-resistant data.

For teams using MailTester’s bulk verification or API, these checks happen at scale without added complexity. Every domain in a received chain gets evaluated for authenticity, so you’re not just filtering dead addresses — you’re filtering compromised or rogue infrastructure.

How to test for malicious routing in your email list using MailTester

You can detect malicious email routing via multiple Received lines from unverified proxies by uploading your list to MailTester’s bulk verification tool, enabling full header analysis to uncover suspicious multi-hop routing, and then filtering and removing any addresses flagged as risky or invalid. This process helps identify compromised or spoofed addresses before they damage your sender reputation.

Step-by-step verification process

Upload your list to MailTester’s bulk verification tool. This scans each address for validity, catch-all status, and known risk signals, including inconsistencies in routing behavior. Most lists contain 5–15% invalid or high-risk addresses, which can skew deliverability if left unchecked.
Enable full header analysis during the scan. This feature examines the full email header chain and flags addresses that show multiple Received lines from unverified or suspicious proxy servers—common signs of compromised accounts or automated abuse. The presence of unexpected hops, especially via non-standard IP ranges or known data centers, increases risk of abuse detection.
Review results and filter by invalid or risky status. Addresses showing multi-hop routing from unverified locations appear in the report with a clear risk indicator. These signals align with industry standards for identifying potential spoofing or hijacking, as defined in RFC 5322 section 3.6, which outlines how Received headers should track legitimate email pathing.
Remove or quarantine flagged entries. Sending to addresses with questionable routing history increases your risk of being flagged by spam filters, especially by large providers like Gmail and Outlook. Proactively cleansing your list reduces bounce rates and protects sender reputation.

Why this matters for deliverability

Even one address with malicious routing can trigger a block or trigger a sender reputation penalty over time. MailTester’s verification process doesn’t just check syntax—by analyzing real-world delivery pathing, it surfaces entries that appear clean but are linked to abuse. This is especially critical in high-volume campaigns where reputation damage occurs slowly but is hard to recover from.

With a 98.9% accuracy rate and credits that never expire, MailTester lets you continuously clean lists without worrying about wasted sends. Run a test on a batch of 100 emails for free at no risk—just check one address at a time using the email checker to preview the system before scaling.

Why filtering proxy routes improves deliverability and sender reputation

You can significantly improve inbox placement and avoid blacklisting by filtering out email addresses routed through unverified proxies. These routes often lead to disposable domains, role accounts, or greylisted servers—sources that correlate strongly with spam complaints and reputation damage. Removing them early reduces bounce rates and keeps your sender reputation healthy.

Proxy routes signal high-risk delivery paths

When an email travels through multiple received lines from unverified proxies, it often passes through infrastructure designed for ephemeral or automated use. These can include public anonymizers, temporary inbox services, or servers with weak verification practices. Such paths are commonly used by spammers or malicious actors to obscure their origin, making them red flags for receiving mail providers.

Spam filters and anti-abuse systems track these patterns. A message arriving via a route with multiple unverified hops is more likely to be flagged or quarantined—even if the recipient is valid. You’re not just sending to a bad address; you’re sending via a route that looks like abuse. That harms your sender reputation, even if your content is clean.

Validating routing improves inbox placement

Let’s be clear: not all proxies are malicious. But unverified ones are a statistical risk. Addresses using such infrastructure frequently belong to disposable domains (like mailinator.com), role accounts ([email protected]), or greylisted servers (those not on a standard DNSBL but still known for erratic behavior).

These types of addresses are overrepresented in spam trap lists and complaint databases. Engaging with them—sending to them, even if they don’t reject—can trigger reputation penalties. Filtering them out before sending reduces both bounce rates and the chances of being labeled a spam source.

Tools like MailTester’s real-time verification API check for routing risks alongside standard validity. The system detects multiple received lines and flags them as high-risk, helping you avoid sending to addresses that route through proxy infrastructure with known abuse patterns.

For example, some ISPs and enterprise mail servers block messages that show signs of routing through unverified third parties. By filtering these early, you avoid wasted sends and keep your sender reputation intact—especially important when scaling campaigns.

Industry standards, such as those outlined in RFC 5321, emphasize proper SMTP routing and sender identity. While not explicit about proxies, the protocol assumes integrity in the delivery path. When that integrity breaks, so does trust.

Use MailTester’s bulk verification to scan entire lists for routes that show suspicious patterns. The service doesn’t just check if an email exists—it checks how it got there.

Real-world examples: When Received line chains expose scam campaigns

Multiple Received lines from unverified proxies—especially across expired domains or unrelated networks—are a red flag for malicious email routing. In real campaigns, these chains often span dozens of hops, bypassing SPF and DKIM checks. MailTester's validation engine detects such routes by analyzing DNS alignment and hop legitimacy, flagging them as high-risk before they reach inboxes.

Case 1: A 'trusted' vendor with 17 unverified hops

Let’s say you get an email from a vendor you’ve worked with for years. It arrives with 17 Received lines—each from a different server, many referencing domains that expired over three years ago. That’s not normal deliverability. That’s a spoofed trail. The routing path has no consistent SPF alignment, and the domains involved show no valid DNS records today. MailTester flags this as high-risk because each hop must pass DNS validation; when the chain breaks, so does trust.

Case 2: A ‘newsletter’ sending across 8 unowned domains

Another case involved a so-called "subscriber list" showing 12 hops through 8 domains—not one owned by the sender. The sender claimed it was a "trusted content partner," but the Received lines showed no record of prior legitimate communication. Some domains were never used for mail, others had no MX records. This route wasn’t just inefficient—it was a clear sign of abuse. MailTester's system identifies such patterns using a mix of DNS, SPF, and behavioral analysis. The longer the chain, the higher the risk of spoofing or hijacking.

These examples highlight why Received line chains matter. They’re not just metadata. They’re a forensic record of where an email traveled. And when that path is built on expired or unverified infrastructure, it’s almost always malicious. According to RFC 5322, Received headers should reflect the actual path of the message. When they don’t, it means something is off.

It’s not enough to validate the final recipient address. You need to inspect the full journey. That’s why we built inbox placement testing with MailTester’s verification engine. Test your email routes before sending to catch routing loops, proxy abuse, or compromised infrastructure. It’s a simple step that prevents delivery failures and protects your reputation.

How to integrate MailTester for continuous list hygiene

You can stop invalid and risky addresses from poisoning your campaigns by using MailTester’s API to validate every new signup in real time, sync verified emails with Mailchimp, HubSpot, Klaviyo, or SendGrid to auto-remove bad data during sync, and run monthly bulk verifications to catch new proxy abuse patterns before they hurt deliverability. This keeps your list clean and your sender reputation intact.

Validate new inputs instantly

Use the MailTester API to check every new email address as it enters your system—before it ever hits a campaign.
Set up real-time validation on your forms, CRM, or signup workflows. Let’s eliminate invalid entries at the source.
Our API returns detailed results, including whether an address is valid, a catch-all, or flagged as risky—because some domains route emails through unverified proxies.

Sync verified data automatically

Connect MailTester directly to Mailchimp, HubSpot, Klaviyo, or SendGrid via our integrations to ensure only clean, deliverable addresses make it into your campaigns.
During sync, the system automatically excludes invalid, risky, or role-based emails—no manual cleanup required.
Pro tip: This prevents accidental exposure to abuse patterns like malicious routing via unverified proxies, which can trigger spam filters and damage your sender reputation.
Learn more about how routing abuse affects deliverability through industry-standard RFC 5321 and RFC 5322 definitions, which define how email should be transmitted and authenticated (IETF).

Schedule monthly deep cleans

Run a full list verification every month using MailTester’s bulk verification tool to detect dormant, invalid, or proxy-used addresses.
Look for clusters of addresses with multiple received lines from unfamiliar or unverified proxies—this is a red flag for abuse.
Monthly checks uncover emerging patterns early, before they get flagged by ISPs or land you on blocklists.
Keep your inbox placement high by acting on insights before harm spreads.

Final takeaway: Proactive verification prevents reputation exposure

Malicious email routing through unverified proxy chains is a detectable threat. These proxy chains often leave multiple Received headers that reveal suspicious path deviations — indicators a message was rerouted through unknown or untrusted infrastructure.

How MailTester stops it

Our full header analysis detects these inconsistencies in real time. By flagging emails that traverse unverified proxies or show abnormal routing patterns, MailTester isolates risky senders before they harm sender reputation or trigger spam filters.

A clean email list doesn’t just confirm deliverability — it validates the journey. You need to know not just if an email is valid, but whether it arrived via a trusted path.

Keep reading

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

Frequently asked questions

Can a single Received line indicate malicious routing?

Yes, especially if it points to an unverified domain with no MX record or a non-routable IP. Multiple hops compound the risk.

Do all emails with multiple Received lines come from malicious sources?

No, but an unexplained chain of hops from unrelated domains is highly unusual for legitimate senders and should be investigated.

How does MailTester verify Received lines without seeing the full message?

The verification API accepts full email headers, including Received lines, to analyze routing paths and check DNS records.

Is it normal for a corporate email to have multiple Received lines?

Yes, but only within the sender’s domain or controlled relay infrastructure. External, unverified hops indicate potential misuse.

Can a proxy with a valid domain still be malicious?

Yes—valid domains can be compromised or used as open relays. Verification relies on DNS, SPF, and routing behavior to confirm safety.

How accurate is MailTester at detecting proxy-based routing?

With 98.9% accuracy, MailTester identifies routing anomalies in Received lines and flags risky or invalid addresses reliably.

Does filtering risky addresses affect my list size?

Yes—risky addresses are typically non-existent, role-based, or proxy-forwarded. Removing them improves list health and deliverability.

Can I automate detection of suspicious Received lines?

Yes—use MailTester’s API or integrations with SendGrid, Klaviyo, or Mailchimp to verify addresses in real time and block risky routes.

Are disposable email providers always caught by verification?

Most disposable domains are flagged as invalid or risky due to lack of domain authority and frequent greylisting.

What happens if I ignore routing anomalies in my list?

You risk higher bounce rates, inbox placement drops, and damage to sender reputation from association with abuse.

Do all verification services analyze Received lines?

No—most only check syntax and deliverability. MailTester includes full header analysis for deeper routing validation.

How often should I check my list for proxy routing risks?

Run bulk checks monthly and verify new entries in real time to stay ahead of abuse patterns.