Reading Headers to Detect Forwarding and Relays in 2026

Why Forwarded Emails Break Delivereability and How to Catch Them Early

You sent an email to what you thought was a real user. It bounced. You checked the address—valid, active, confirmed. But the delivery failed anyway. What went wrong?

Not every bounce comes from a bad address. A growing number of delivery failures come from forwarded or relayed emails—messages that never passed through the original sender, breaking the trust chain email systems rely on. These messages often trigger spam filters, degrade sender reputation, and sink inbox placement. The warning signs? Hidden in plain sight—inside email headers.

Reading headers to detect forwarding and relays isn’t just for IT teams. It’s a practical, preventive step that stops reputation damage before it starts. You don’t need to parse every byte—just recognize the signals that show an email was rerouted, not sent.

Key takeaways

• Forwarded or relayed emails disrupt sender reputation by breaking the end-to-end trust path required by inbox providers.
• Many bounces labeled as "invalid address" are actually caused by undetected forwarding, not address errors.
• Inspecting Received headers reveals relay patterns and forwarding chains, allowing you to flag and filter risky delivery paths early.

What Are Forwarding and Relay Activity in Email Headers?

Forwarding and relay activity in email headers occur when an email is redirected or passed through third-party servers not part of the original sending path. Forwarding happens when a recipient sends an email to someone else without the sender’s involvement, while relay activity means the email was processed by a server that wasn’t meant to handle it. Both alter the original email path, which can break alignment with SPF, DKIM, and DMARC—key email authentication standards.

How Forwarding Distorts the Original Path

When you forward an email, especially in services like Gmail or Outlook, the system adds new headers and often reroutes the message through its own servers. This changes the envelope sender and the visible “From” address, which can disrupt SPF checks that rely on strict sender domain alignment. You might see multiple Received lines in the headers, with newer entries from Google, Microsoft, or another provider, not the original sender’s mail server. As a result, SPF verification can fail even if the email is legitimate.

Why Relay Activity Is a Red Flag for Deliverability

Relay activity typically happens when an email is passed through a server that wasn’t designed to originate messages—like a proxy, shared hosting system, or automated forwarding tool. Unlike forwarding, which is user-initiated, relay activity can signal abuse. Mail servers use headers to trace the message path; unexpected relay hops suggest possible spoofing or compromised accounts. If your email chain shows a relay through a public SMTP server not associated with your domain, it increases the risk of your messages being flagged as spam.

Both forwarding and relay activity are visible in the Received header chain. Each entry shows the date, IP address, domain, and mail server involved. By reading these headers in order, you can spot where an email was rerouted or processed outside the intended flow. Tools like MailTester's email checker can help surface such issues before sending by validating the integrity of addresses and detecting anomalies in email routing patterns. This visibility is crucial for debugging deliverability problems. The IETF's RFC 5322, which defines the standard email format, details how these headers should be structured, making header analysis essential for compliance.

A well-formed email should show a clean, logical path from origin to destination. Extra hops, especially from generic providers like smtp.gmail.com or outlook.com, without clear intent, should raise concern. If you're sending to verified lists, ensure your sending infrastructure avoids these disruptions.

How to Identify Forwarding Using the Received Header Field

You can detect forwarding by examining the Received: header field in an email’s full header. Multiple entries indicate a chain of hops. If the source domain or IP in one entry doesn’t match the sender’s original domain, or if a domain appears random (e.g., mail-relay-789.xyz), that’s a strong sign of forwarding or relay. The pattern Received: from [unknown-or-random-domain] by [relay-server] is common in forwarded messages. Use tools like MailTester's inbox placement test to validate delivery path integrity.

Spot the Signs in the Headers

• Look for multiple Received: entries—each one represents a server the email passed through.
• Check if any from [domain] entry has a domain that doesn’t match the sender’s actual domain or looks like a random string (e.g., mxrelay-12345.com).
• Pay attention to entries that list a relay server (like by smtp-relay.example.com) but have no clear source IP or domain.
• Forwarded messages often include a hop where no sender IP is logged, or the source is a generic internal server, not a known origin.
• Common red flags include from unknown [IP] or from [random-subdomain].com when the message claims to be from a real business.

What It Means for Delivery and Trust

When an email bounces or lands in spam due to a forwarded path, the original sender's reputation is often irrelevant. The relay or forwarding server becomes the effective origin point. This undermines sender reputation and can trigger filters. According to RFC 5322, proper use of the Received: field is critical for traceability and authentication.

Let's be clear: forwarding chains don’t just obscure the path—they also increase the risk of abuse. A relay that lacks proper SPF or DKIM validation can allow spoofed messages to propagate. You’re not just checking headers—you’re validating the sender’s journey.

If you're unsure whether your recipients’ inbox placement is being sabotaged by relay chains, run a test using MailTester’s inbox placement tester to simulate real-world delivery across major providers and detect routing anomalies.

Reading the Received-Line Hierarchy to Detect Relay Servers

Start at the bottom of the email header—where the most recent 'Received:' line appears—and work up. If a server outside your domain appears in the chain unexpectedly, especially one that doesn’t belong to a trusted partner or infrastructure provider, it may be relaying the message. Track the domains in each 'Received:' field; mismatches with your own sending domains are signs of intermediate relay activity, which can trigger spam filters or degrade sender reputation.

The Step-by-Step Process

1. Locate the last 'Received:' line—this is the most recent hop, typically from the receiving server. This is your starting point, the entry point of the message into the recipient’s system. Reading from here lets you trace the path backward with clarity.
2. Move upward through each 'Received:' entry—each line represents a server that handled the message. Pay attention to the sending IP and domain listed in each hop. A change in domain or IP that you don't recognize may indicate a relay or proxy.
3. Compare domains in 'Received:' fields with your sending infrastructure—if a hop shows a server from a domain not in your email ecosystem (e.g., a third-party email service, a corporate SaaS platform, or a public email provider), it could be acting as a relay. This is especially notable if the hop occurs between your outbound mail server and the recipient’s inbox.
4. Identify unexpected or unauthorized relays—if a server with a public or consumer email domain (like Gmail, Yahoo, or a .mailgun.net address) appears in the chain without your knowledge, that’s a red flag. Relay activity from such servers often correlates with poor deliverability.
5. Validate your findings with SPF/DKIM/DMARC—if a relay is misconfigured, the message may fail SPF or fail to authenticate. A header showing a third-party relay might explain a failed authentication check, even if the sender’s domain appears in the 'From:' field.

Why Mismatches Matter

Unexpected relay servers can originate from compromised accounts, poorly configured mailing systems, or even forwarding setups used by spammers. When a message passes through an unauthorized relay, it increases the risk of being flagged as spam. You can't always see the full path, but tracing the 'Received:' chain helps identify where a message may have been rerouted or intercepted.

According to the IETF’s RFC 5322, email headers are designed to record the full message path—making header inspection essential for diagnosing deliverability issues. This is especially important when debugging why a campaign lands in spam folders or fails to deliver entirely.

If you’re managing a high-volume email list, verifying addresses before sending can prevent sending to relay-heavy or compromised inboxes. Use bulk email verification to catch and remove entries that might be proxies, forwarding accounts, or other relay-prone addresses.

What to Look for: Red Flags in the Received Header Chain

You’re looking for anomalies in the email’s journey: multiple hops from unrelated domains or IP ranges, servers with generic or missing hostnames like 'unknown.domain.com', or sudden jumps from a trusted sender to a public relay like Gmail or Hotmail. These signals often indicate forwarding, relaying, or abuse. Let’s break down the key red flags visible in the Received header chain.

Trusted vs. Suspicious Hop Patterns

• Multiple hops from geographically distant or unrelated domains (e.g., a message sent from a company in Germany appearing to pass through a server in Nigeria with no apparent connection).
• Sudden jumps from a known corporate domain to a generic public mail server like Gmail or Outlook without an explicit routing path — common in forwarded messages or compromised accounts.
• One or more servers in the chain with anonymous or malformed hostnames, such as from unknown.domain.com, unknown[IP], or blank entries — a sign of misconfiguration or abuse.
• Repeated or unexpected use of the same relay (e.g., multiple messages from different senders hitting the same public SMTP queue) — often a sign of open relays or spam distribution.

When Relays Break Trust

Relays aren’t inherently bad — they’re part of how email flows across networks. But a relay that appears out of place breaks the expected chain of trust. For example, if a message sent from a verified business domain suddenly shows a Received: from mail-pop3.google.com hop with no prior connection to Google’s infrastructure, that’s a red flag.

According to RFC 5322, email headers should reflect a consistent, traceable path from sender to recipient. When this path contains disconnected or unverifiable hops, it raises suspicion. A single anonymous or generic relay may not block delivery — but multiple such instances across a list reduce sender reputation.

Using a real-time email verification tool can help catch these issues before they damage deliverability. You can test individual addresses or validate entire lists to detect malformed headers, forwarding patterns, or high-risk accounts.

• Verify individual addresses before sending with our email checker to spot risky patterns early.
• Run bulk checks on your list to identify problematic domains, relay-heavy addresses, or forwarding chains with our bulk verification tool.
• Use our inbox placement tester to see how your messages land in real inboxes, including whether they’re being rerouted or marked as suspicious due to header anomalies.

How Forwarding Affects Sender Reputation and Inbox Placement

Forwarded emails often carry altered or missing authentication headers, which can cause DMARC failures and lead inbox filters to mark them as suspicious. Relay activity—especially when linked to shared or compromised IP addresses—signals reputational risk, reducing the likelihood of inbox placement even for legitimate senders. A single forwarded message with unauthorized relays can trigger suspicion across entire domains.

Authentication Breaks in Forwarded Messages

When an email is forwarded, especially through tools like Gmail or corporate relays, the original authentication tags (SPF, DKIM, DMARC) can become invalid or mismatched. The email now appears to come from a different IP than the one authorized by SPF, or the DKIM signature fails because the message body was altered during forwarding.

DMARC checks depend on all authentication layers passing. If a forwarded message fails any one, it gets rejected or quarantined—regardless of its content. This is why even a legitimate email from a trusted sender can land in spam if routed through a relay that doesn’t preserve the integrity of the authentication chain.

Relay Activity and Sender Reputation

Relays that handle large volumes of email—especially shared ones, like public forwarding services or compromised systems—often have poor sender reputations due to misuse by spammers. When your email passes through such a relay, ISPs assume your message may be equally risky.

The more frequently a recipient’s inbox encounters emails from known relay sources, especially those with poor reputations, the more likely future messages from that domain are to be filtered. Even if your own sending practices are clean, a forwarded path through poor-quality relays can indirectly damage your sender reputation.

For example, RFC 5322 and RFC 5321 define how email should be constructed and routed. When forwarding software doesn’t follow these standards—especially by failing to preserve authentication headers—it undermines the entire trust mechanism behind email deliverability (RFC 5322; RFC 5321).

Let’s be honest: you can’t control every relay a user’s message passes through. But you can reduce the risk by catching invalid or high-risk addresses before they’re sent. Tools like MailTester’s email checker verify whether an address is likely to forward, relay, or result in a bouncy, spoofed, or spoof-like delivery path.

Using Real-World Headers: A Practical Demonstration

Let’s walk through a real example where headers reveal a forwarded email that wasn’t properly authenticated. An email from [email protected] arrives via mail-relay-113.example.net, but the Received: chain shows no path back to acme.com’s own mail servers. This lack of direct routing — combined with no SPF/DKIM authentication in the chain — signals a relay or forwarder, not a direct send. This pattern is a red flag for spoofing or poor mail configuration.

Step-by-Step Header Analysis

1. Locate the original Received: header at the top of the full header output. This is the first hop into your mail system. In this case, it points to mail-relay-113.example.net. This is not acme.com’s mail server, so the message didn’t leave from there directly.
2. Trace the chain backward using the order of Received: lines. You’ll find no entry that links to acme.com’s MX or SMTP server. If the sender were legitimate and sending directly, the chain would end at acme.com’s infrastructure.
3. Check for SPF alignment. Look at the Received-SPF result in the headers. If it shows none or fail at the final hop (and no prior valid SPF checks), that’s a strong signal that the sender’s domain is not authorized to send through this relay.
4. Inspect DKIM signatures. If the message lacks a valid DKIM signature from acme.com, or if the signature is tied to a different domain (like example.net), that confirms the message was relayed or modified.
5. Look for Resent-* headers. These are definitive indicators of forwarding. If you see Resent-From: or Resent-Date:, the message was intentionally re-sent — likely through an automated forwarder or a misconfigured alias. This breaks the chain of direct delivery.
6. Validate the IP's reputation. Query the relay’s IP (192.0.2.42) using public tools like MxToolbox or Spamhaus. If it’s listed in a blocklist or known for spam, it further confirms relay abuse.

Why This Matters for Deliverability

Misconfigured forwards or relays often appear in header chains without proper authentication. This can lead to delivery failures or spam filtering — especially if the forwarder has poor sender reputation. The RFC 5322 standard requires that email headers reflect the actual path of delivery. When they don’t, it raises red flags.

If you’re verifying a list before sending, you can prevent these issues early. Use MailTester’s bulk verification tool to catch forwarders and relay-like patterns before they impact your deliverability. You’ll catch invalid or misrouted email patterns before they hit your inbox.

How MailTester Helps Clean Lists of Forwarded or Relayed Addresses

You can detect forwarded or relayed email addresses by analyzing the message headers during delivery — but doing it at scale manually is impractical. MailTester automates this: its bulk verification finds risky, catch-all, and invalid addresses, while its real-time API and inbox-placement testing pinpoint forwarding anomalies that hurt deliverability. This means you’re not just checking if an address exists — you’re checking if it lands in the inbox.

Bulk Verification Catches Hidden Forwarding Risks

When you run a list through MailTester’s bulk verification, it doesn’t just return “valid” or “invalid.” It flags addresses with suspicious patterns — like consistent relay behavior in header histories or catch-all responses that suggest mail is being redirected. These are red flags that the address might be part of a forwarding chain, which increases bounce risk and can trigger spam filters.

For example, an address that routes through multiple domains or has a consistent “Received” header chain from unexpected servers is likely a relay. MailTester’s system detects such paths during verification and marks them as “risky,” so you don’t waste sends on addresses that will never reach a real inbox.

Real-Time Checks and Inbox Testing Confirm Delivery Impact

Using MailTester’s real-time API, you can test individual addresses as you collect them — catching forwarding anomalies before they ever hit your queue. The API checks header trails during actual transaction attempts, spotting relay paths or shared mailbox behaviors that signal a high chance of delivery failure.

Go further with inbox-placement testing, which simulates sending to real inbox folders. If a forwarded or relayed address routes through a third-party gateway, inbox testers often fail to place the message. MailTester’s inbox tester helps you understand how much more likely those addresses are to get filtered or blocked — not just bounced.

For those syncing with tools like Klaviyo, HubSpot, or SendGrid, MailTester’s integrations help you apply these checks automatically. You can even verify a list before sending, using the email checker tool on single addresses or a full batch via bulk verification.

What Each Verification Verdict Tells You About Relay Risk

Each email verification verdict from MailTester signals something real about relay risk. A valid address means it’s receptive and wasn’t flagged for forwarding patterns. An invalid one suggests the mailbox doesn’t exist—possibly a black hole or a dead end in a relay chain. Catch-all addresses resolve any input, making them high-risk for abuse and relay misuse. If the result is risky, we detected actual forwarding or relay activity, even if the address is technically valid. Let’s break down what each outcome means in practice.

Understanding the Verdicts

Understanding these verdicts isn't just about syntax—it's about spotting infrastructure signals in real time. You can’t rely on address presence alone to judge deliverability. Forwarded emails often trigger greylisting, bounce loops, or even being flagged as spam.

Relay abuse often hides in plain sight. A valid-looking address can still be forwarding to a spam trap or a black hole. The real danger isn’t just the address—it’s the behavior behind it. Tools like MailTester probe for these signs using SMTP, header analysis, and historical bounce telemetry. You can’t detect relay behavior from the address alone; you need the full context.

For example, a domain that returns “valid” for any input (a catch-all) isn’t just permissive—it’s enabling abuse. This is why we flag it. Similarly, a single address that repeatedly bounces or triggers greylisting across multiple sends can be a sign of forward loops, common in relay setups.

For deeper insight into how mail systems behave under real-world conditions, see the SMTP specification (RFC 5321), which governs how messages are routed. It defines the expected server roles and behavior—deviations from this guide you toward relay patterns.

Integrating Header Checks into Your List Hygiene Routine

You can catch forwarded and relayed addresses early by checking headers during list hygiene. These patterns often signal invalid, spoofed, or high-risk recipients. Include header analysis in your pre-send checks to reduce bounces, improve sender reputation, and avoid inbox placement issues. Tools like MailTester automate this, scanning real delivery paths to surface risky addresses before you send.

Pre-flight checklist: Validate with headers

• Run a header analysis on every new email list before sending.
• Use MailTester’s bulk verification to test hundreds of addresses and flag those with forwarding or relay patterns in their delivery path.
• Check for repeated use of relay domains (like forwarding-service.com or relaymail.net) across your logs—they’re red flags for abuse or spoofing.
• Filter out any address linked to a known relay or proxy server using tools that inspect actual delivery routes, not just syntax.
• Review header chains in bounced messages to trace where forwarding happened; repeat patterns point to systematic misuse.
• Monitor your own outbound logs for unexpected relay hops—especially when sending to domains you don’t typically use.

Automate detection to stay ahead

Manual header review is slow and error-prone. Let tools like MailTester do the heavy lifting. Our email verification API checks headers in real-time during list cleansing, identifying risky accounts you’d miss otherwise. It’s not just about validity—it’s about context.

Relay and forwarding patterns can hide behind valid domains. For example, an address may pass syntax validation but still route through an open relay, increasing its risk profile. The RFC 5322 specification outlines how email headers should be structured; violations or anomalies in the Received: field are common signs of non-compliant or suspicious routing.

When you see the same relay domain show up repeatedly in headers—especially with low sender reputation—treat those addresses with caution. Some attackers use relays to bypass SPF and DKIM checks. A 2023 report from the Anti-Phishing Working Group noted that phishing campaigns often leverage third-party relays to disguise origin points. Identifying these early helps stop your list from being used as a vector.

Let’s be clear: no single tool catches everything. But combining header checks with accurate list validation reduces your risk. Use MailTester’s inbox placement testing for real-world delivery proof—this includes testing how your message routes through real ISP filters, which detect relay-heavy paths.

Bottom Line: Headers Are Your First Line of Defense Against Delivery Failures

Even a valid email address can fail to deliver if it’s set up to forward or relay messages through third parties. These paths introduce delays, authentication issues, and exposure to spam triggers.

Examining the Received: header chain exposes hidden relay points, misconfigured servers, and domains with poor sender reputations—issues that can silently undermine inbox placement.

Prevent these risks before they impact your campaign. With MailTester’s 98.9% accurate verification, you can clean your list and validate sender reputation at scale.

Sources

• The number of top domains at DMARC enforcement grew from 233,249 in 2023 to 411,935 in 2026 — a 77% increase driven largely by mailbox-provider sender mandates. — EasyDMARC 2026 DMARC Adoption & Enforcement Report (2026)
• Since May 5, 2025, Microsoft Outlook requires SPF, DKIM, and DMARC from domains sending 5,000+ emails per day, rejecting non-compliant mail outright at the SMTP level with error 550 5.7.515. — Microsoft Outlook requirements (via MailOver bulk-sender requirements guide) (2025)

Keep reading

• Email authentication: SPF, DKIM, DMARC, BIMI and MTA-STS (complete guide)
• Impact of Non-UTF-8 Character Sets on DKIM Canonicalization in 2026
• Email Authentication Checker with DKIM SPF Alignment Analysis
• Why Some Email Gateways Alter MIME Boundaries and Cause DKIM Mismatch
• Microsoft Finally Sends DMARC Aggregate Reports — What Changed

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

Frequently asked questions

Can forwarded emails bypass spam filters?

Yes — forwarded messages sometimes pass spam checks if they arrive from a trusted domain, but they still carry red flags when headers reveal relay activity.

How do relay servers affect DMARC authentication?

Relay servers often lack proper DMARC alignment, causing authentication failures. MailTester flags such addresses as risky.

Is it normal to see multiple Received: headers?

Yes — every server that handles an email adds a Received: line. But unexpected domains or IPs signal forwarding or relay misuse.

Can MailTester detect forwarded emails in real time?

It doesn’t inspect headers directly during verification, but it flags addresses with patterns consistent with forwarding or relay abuse.

Why does a catch-all address appear risky?

Catch-alls accept all emails regardless of validity, making them common in forwarding loops and relay abuse. MailTester marks them as risky.

How accurate is MailTester's verification process?

MailTester achieves a 98.9% accuracy rate using real-time SMTP checks, DNS analysis, and behavioral pattern detection.

What happens if I send to a relayed address?

The email may be blocked by spam filters, flagged as suspicious, or never reach the intended inbox due to failed authentication.

How can I improve deliverability using header inspection?

By identifying forwarded or relayed paths early, you reduce the risk of reputation damage and increase inbox placement accuracy.

Do all forwards appear in the header chain?

Yes — any relay or forward leaves a trace in the Received: field. If the chain is altered, it’s visible in raw headers.

Can I automate header-based verification?

Yes — MailTester’s API and integrations with Mailchimp, SendGrid, and HubSpot enable automated list hygiene at scale.

Are disposable domains always forwarded?

No — disposable domains are often used for one-time forwards, but they don’t inherently indicate relay activity. MailTester detects them separately.

How often should I check headers for relay signs?

Inspect headers on delivery failures or high bounce rates. Use MailTester to proactively test your list before sending campaigns.