Why are emails not being delivered when I include my email signature?
Published 23 May 2025
Updated 26 Sep 2026
14 min read
Summarize with
Updated on 26 Sep 2026: We expanded the troubleshooting steps for reply chains, embedded images, Microsoft 365 trace results, and DKIM signing order.
If an email delivers without your signature but is rejected, quarantined, or placed in junk when the signature is added, the signature has changed something that a sending system or receiving filter does not like. The usual causes are a URL or redirect in the signature, a linked image host, an embedded image payload, malformed HTML or MIME, or a server-side signature process that changes the message after DKIM signing.
Treat this as a content-dependent delivery failure first, not as a mystery DMARC problem. The fastest path is to prove whether the message leaves your mail platform, identify whether it was rejected or filtered after acceptance, then remove signature elements one by one until the failing part is isolated.
- Most likely cause: a link, tracking URL, social icon, image host, CTA domain, or embedded image is triggering filtering.
- First test: send the same message to several recipient domains with and without the signature.
- Second test: check message trace, bounces, quarantine, and authentication results before editing DNS.
- Best fix: rebuild the signature in simple HTML and add each element back after a clean delivery test.
The short answer
Your signature is part of the message body, not decoration outside it. That body can include external links, hosted or embedded images, tracking parameters, hidden HTML, and brand domains that have their own reputation. A filter can accept the plain email but reject, quarantine, or place the signed version in junk because the signed version has a different content and reputation profile.
Do not start with DNS changes
When the same sender, recipient, subject, and message body deliver only after the signature is removed, the signature content is the main suspect. DNS changes come later, after you confirm whether SPF, DKIM, and DMARC pass on the signed version.
- Ask for a bounce: if one exists, the SMTP status and enhanced code can identify content, policy, size, or reputation rejection.
- Check trace: message trace confirms whether your mail platform blocked, deferred, or handed off the message.
- Compare versions: one message without the signature and one with the signature gives a clean test pair.
- Save headers: received headers and authentication results show what changed at delivery time.
The most revealing clue is whether the message works when sent to your own mailbox but fails for a client. Internal success proves only that your internal route accepted the message. External mail can pass through a different outbound route, signature process, and recipient filter, so trace both paths before assigning the cause.
Signature content problem
- Pattern: plain message delivers, signed message fails.
- Cause: URL, CTA, social icon, image host, embedded image, or HTML.
- Fix: strip the signature, add elements back, and retest each version.
- Evidence: delivery changes only when the message body changes.
Authentication problem
- Pattern: failures appear across many messages, not only signature messages.
- Cause: SPF, DKIM, DMARC, forwarding, or a message rewrite after signing.
- Fix: verify DNS records, signing order, and the domain used in visible From.
- Evidence: headers show failed authentication or a strict policy rejection.
Why the signature changes the result
A small signature can add more risk signals than the email body itself. A recipient filter reads the full MIME message, checks reputation clues, and evaluates the HTML. It sees that the message now contains more domains, more code, remote requests, or extra MIME parts.
Email signature deliverability infographic showing HTML, links, image hosting, and recipient filtering.
A CTA link is a common trigger. A button for a report, booking page, social profile, or tracking page can point at a domain that has weak reputation or appears on a blocklist (blacklist). URL shorteners and multi-step redirects add more domains for a filter to assess. The sender domain can be clean while the linked domain harms the message.
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|
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|---|---|---|
CTA link | URL reputation or redirects | Remove link |
Logo | Remote host, MIME part, or payload size | Use text only |
Phone | Content pattern | Remove number |
Social | Extra URLs | Remove icons |
HTML | Bad markup or excess code | Simplify code |
Common signature elements and the first thing to test.
Do not assume a text-only-looking signature is actually text only. Signature editors can generate nested tables, inline CSS, tracking links, remote images, hidden preview text, and copied word-processor markup. Embedded images add MIME parts and encoded bytes, and some systems treat them like attachments. Check the source, image dimensions, file size, and total transmitted message size.
Test it without guessing
Use a controlled test because random resend attempts hide the cause. Send one clean message without the signature, then send the same subject and body with the signature. Use a real receiving mailbox and inspect the result with an email tester so the headers, authentication results, and content warnings sit in one place.
- Baseline: send a plain version to your own account, a Gmail account, and the affected client domain.
- Full signature: send the same message with the complete signature and compare the result.
- Text only: remove all links, images, social icons, and buttons, then test the name and title only.
- One element: add back the phone number, then the website, then the CTA, then each social link.
- Final proof: repeat the failing test twice before treating the isolated element as the cause.
Email tester
Send a real email to this address. Suped shows a results button when the test is ready.
?/43tests passed
If the failure starts when you add one specific link, remove it or use a direct URL on a clean branded domain without shortening or an unnecessary redirect. If it starts when you add a logo, compare a small remote image on a reliable HTTPS host, an embedded image, and a text-only version. If it starts when the HTML version returns, rebuild the signature in plain, minimal HTML.
Minimal HTML signature for testingHTML
<table role='presentation' cellpadding='0' cellspacing='0'> <tr> <td style='font:14px Arial,sans-serif;color:#222;'> Jane Smith<br> Acme Co.<br> example.com </td> </tr> </table>
This test signature is intentionally plain. Start with text and one branded link, prove delivery, then add design only when each added element keeps passing.
Check where the signature is added
The insertion point matters as much as the signature content. A mail client usually adds the signature before submission, so the sending service signs the completed body. A transport rule, gateway, or server-side signature process can add or rewrite content later. If that happens after DKIM signing, the receiver can calculate a different body hash and return DKIM failure.
Client-side signature
- Added: before the message reaches the sending service.
- DKIM: normally signs the completed message body.
- Test: compare desktop, webmail, and mobile submissions.
Server-side signature
- Added: by a rule, gateway, or service during mail flow.
- DKIM: fails if signing happens before the final body change.
- Test: inspect the final received source and message route.
Sign only after the final rewrite
Place DKIM signing after every process that changes the body. If that order cannot be changed, configure the final outbound system to add a valid DKIM signature whose signing domain meets the configured DMARC domain-matching mode for the visible From domain.
Check replies, forwards, and duplicate signatures
A new message can deliver while a reply or forward fails. Long threads can repeat the logo, social icons, tracking links, and embedded image parts each time someone replies. That increases the transmitted message size and the number of URLs a filter evaluates, even when the newest reply contains only one sentence.
- Start a new thread: send the same text and signature outside the existing conversation to separate thread size from signature content.
- Count signatures: check whether a client signature and a server-side rule both add the same block.
- Inspect MIME parts: look for repeated inline images or attachments with new content IDs on every reply.
- Scope mail-flow rules: use a reliable marker or exception so a disclaimer is not appended repeatedly.
Test both a new message and a reply
If the new message passes but the reply fails, trim the quoted thread and remove repeated signatures before changing DNS. If both fail when the same signature is present, continue isolating links, images, HTML, and signing order.
Check authentication before blaming the receiver
Signature content and authentication are separate, but they can collide. If a signature is added after DKIM signing, the message body changes and DKIM can fail. If SPF also fails to pass with a domain that matches the visible From domain under DMARC, DMARC fails and an enforcing receiver can reject or quarantine the message.
Start with a domain health checker to confirm your SPF, DKIM, and DMARC records are valid. Then use ongoing DMARC monitoring to see whether the failing mail stream passes authentication at real receivers and whether the authenticated domain matches the visible From domain under DMARC.
Issue steps to fix dialog showing the issue overview, tailored fix steps, and verification action
Suped is our DMARC reporting and email authentication platform. For this problem, compare authenticated sources with unverified sources, filter failures by sender, and use the issue steps to fix broken SPF, DKIM, or DMARC records after the signature tests show an authentication fault.
Basic DMARC record for monitoringDNS
v=DMARC1; p=none; rua=mailto:dmarc-reports@example.com
Where Suped fits
Use Suped to verify whether authentication changed between the working and failing mail streams. The controlled message tests still isolate the signature element, while DMARC reports and SPF or DKIM diagnostics show whether a sending source or signing change needs attention.
- Detection: issue detection flags the failing source or record.
- Action: fix steps identify what to change in DNS or the sending platform.
- Comparison: source-level results separate a content trigger from an authentication failure.
- Follow-up: alerts show whether authentication or reputation changes after the fix.
Check the domains inside the signature
A sender domain can have good reputation while a linked signature domain has poor reputation. That includes a CTA landing page, social redirect, URL shortener, logo host, or old domain that inherited abuse history. Check every domain that appears in the signature, including redirect destinations, not only the domain in the From address.
For reputation checks, use blocklist monitoring and blacklist checks for the sending IP, sender domain, landing page domain, and image-hosting domain. A single blocklist or blacklist entry does not prove rejection, but it gives you a specific lead to compare with the bounce, trace, and test results.
Signature risk triage
Prioritise the behavior and reputation of each element. Risk does not depend on the number of elements alone.
Low
No external assets
Plain name, role, and direct branded link.
Medium
Known domains only
Known image host, phone number, social links, or tracking tags.
High
One risky element
URL shortener, redirect chain, poor-reputation domain, or broken HTML.
Pay attention to links that look branded but redirect somewhere else. A filter can score the first URL, each redirect, the final destination, and the visible link text. Keep the visible domain and destination domain consistent wherever you can.
Content clues that matter
- Mismatch: button text says one domain while the target redirects to another.
- Remote image: the logo loads from a host with weak reputation, broken HTTPS, or blocked hotlinking.
- Tracking: long parameters and several redirects make a normal CTA resemble a bulk marketing URL.
- HTML noise: nested tables, hidden spans, and copied editor markup add code that the signature does not need.
Trace Microsoft 365 delivery accurately
With Microsoft 365, check message trace first, but read the event details rather than relying on the summary status. Inside Microsoft 365, Delivered can include mail placed in Junk or quarantine. For an external recipient, confirm whether Exchange Online handed the message to the recipient's server, then ask the recipient to check quarantine and gateway logs. A successful external handoff does not prove inbox placement.
If trace does not show an external handoff, inspect outbound spam policy, mail-flow rules, malware scanning, and data loss prevention rules that evaluate the body, attachments, message size, or URL patterns. Compare the rule events for the signed and unsigned test messages.
Gmail can also be useful as a test recipient because it often exposes visible warnings or clearer authentication details in message headers. A public Gmail support thread describes the same class of symptom: a signature link changes delivery behavior.
If internal delivery works
- Meaning: your internal route accepted the message, though it can still be in Junk or quarantine.
- Next: send to several external domains and compare failures.
- Check: event details, headers, bounce codes, and linked-domain reputation.
- Risk: the external path has stricter content or reputation filters.
If internal delivery fails
- Meaning: your own platform is likely blocking, modifying, or filtering the message.
- Next: check mail-flow policy, malware scanning, and transport rules.
- Check: whether a server-side signature process rewrites the body after signing.
- Risk: your own mail controls are acting before final delivery.
This is also why an email can deliver internally but not to external recipients. Compare the accepted internal copy with the external failure, inspect the final headers, and confirm what the recipient's filtering system did with the message.
A practical fix order
Use the order below when you need to fix this quickly. It avoids changing too many variables at once and keeps the investigation tied to evidence.
- Confirm delivery state: find out whether the recipient got a bounce, junk placement, quarantine, or no visible copy.
- Trace the message: confirm whether your platform sent the message outside your organization and which policy events ran.
- Strip the signature: send text only, then add back the phone, domain, CTA, image, and social links.
- Test a clean thread: compare a new message with a reply so repeated signatures and quoted content do not hide the cause.
- Inspect headers: compare SPF, DKIM, and DMARC results between passing and failing versions.
- Rebuild simply: replace copied editor HTML with clean HTML and one branded destination domain.
- Monitor after fixing: watch authentication, rejection patterns, and blocklist or blacklist changes.
Troubleshooting flowchart for email signature delivery failures, including trace, links, authentication, and HTML.
Once you find the failing element, keep the fix plain. Use one branded domain, keep social icons out of early tests, avoid URL shorteners, host small images on a reliable HTTPS domain, and keep HTML lean. A reliable, minimal signature is the goal.
Views from the trenches
Best practices
Test the same email with and without the signature across several recipient domains.
Remove signature elements one at a time so each test has one clear changed variable.
Keep signature HTML simple and host linked images on a clean branded HTTPS domain.
Check message trace before changing DNS, because trace shows where delivery stopped.
Common pitfalls
Assuming a visible text signature has no hidden HTML, tracking, or remote images.
Checking only the sender domain while ignoring CTA, image, and redirect domains.
Changing SPF or DMARC records before proving authentication actually failed in headers.
Testing only internal mailboxes, which can miss recipient-side content filtering.
Expert tips
Use a plain-text baseline first, then add the phone number, URL, CTA, and logo back.
Compare headers from passing and failing versions to catch body rewrites after DKIM.
Treat blocklist and blacklist results as leads, then confirm with bounces and trace.
Keep one production-safe signature version ready while testing a rebuilt HTML version.
Marketer view
Marketer from Email Geeks says the first question is whether the message is bouncing, landing in spam, or vanishing before the recipient can see it.
2021-11-26 - Email Geeks
Marketer view
Marketer from Email Geeks says a phone number or domain inside a signature can be enough to change filtering, so both should be tested separately.
2021-11-26 - Email Geeks
Confirm the fix
Retest the final signature in a new message and in a reply. Send both to more than one external recipient domain, then confirm the message reached the intended folder rather than relying on the absence of a bounce.
Inspect the final received headers after every server-side signature or disclaimer has been added. DKIM should pass on that final body, and either SPF or DKIM must pass with a domain that meets DMARC's matching requirement for the visible From domain.
Use Suped's DMARC reports and SPF or DKIM diagnostics to confirm the sending source stays authenticated after the fix. Pair that evidence with message trace and blocklist or blacklist monitoring when delivery changes again.
