Why are emails not being delivered when I include my email signature?
Matthew Whittaker
Co-founder & CTO, Suped
Published 23 May 2025
Updated 19 Aug 2025
9 min read
It can be incredibly frustrating when you send an important email, only to find out it never reached the recipient. The confusion compounds when you realize the email delivers perfectly fine as soon as you remove your email signature. This is a surprisingly common issue, and it points to a deeper problem than just a formatting glitch.
Email signatures, while seemingly innocuous, can sometimes trigger spam filters or even cause your emails to be silently dropped by recipient mail servers. This happens because certain elements within your signature might be perceived as suspicious or non-compliant with email sending best practices. Let's explore the common culprits and how to resolve them.
Content and formatting pitfalls
One of the most frequent reasons an email signature causes delivery problems is due to its content, especially if it includes complex elements. Mailbox providers (like Google and Microsoft) are constantly on the lookout for suspicious patterns that indicate spam or phishing attempts. A signature with too many links, poorly formatted HTML, or images hosted on untrustworthy domains can easily trip these filters. Even a legitimate link to a domain with a low reputation can lead to delivery issues. Complex signatures with links and images hosted in untrustworthy environments are a common problem.
Images within signatures, while visually appealing, can also pose problems. If images are embedded incorrectly, are too large, or are hosted on unreliable servers, they can cause the email to be flagged. Sometimes, email clients (like Outlook or Gmail) may even convert images into attachments if the email format isn't strictly HTML, which can be seen as suspicious. Signatures can impact the ability to send mail, particularly when they contain elements that deviate from standard email practices.
Risky signature elements
Excessive links: Too many URLs, especially to various social media profiles or promotional content, can appear spammy.
Poor domain reputation: If any domain linked in your signature (e.g., your website, social media, or image host) has a poor sender reputation or is on a blocklist, it can negatively affect your email's deliverability. Even if the links themselves are legitimate, the associated domains might be compromised or have a history of spam.
Large or unoptimized images: Heavy image files can slow down loading and may be interpreted as suspicious, especially if not properly linked or embedded.
Complex HTML/CSS: Overly intricate or poorly coded HTML can be seen as an attempt to hide malicious content, leading to filtering or even outright rejection.
Another subtle but significant factor is the email format. Emails can be sent in plain text, HTML, or rich text. If your signature relies on styling, images, or hyperlinks, but your email client sends the message in plain text, these elements either won't display correctly or, worse, could be stripped out or presented as suspicious attachments. This mismatch can confuse recipient servers and lead to deliverability issues. Always ensure your email client is configured to send messages in HTML format for signatures with rich content.
Email authentication and blocklists
Email authentication protocols are crucial for verifying that an email is legitimate and has not been tampered with. These include Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting, and Conformance (DMARC). While email signatures themselves don't directly cause authentication failures, their content, particularly external links or images hosted on different domains, can complicate the authentication process. If your signature includes elements that point to domains not covered by your SPF record, or if it modifies the email content in a way that invalidates the DKIM signature, it could lead to delivery problems. Understanding how signatures can cause SPF or DKIM failures is key.
Mailbox providers often consult various blocklists (or blacklists) to determine if an incoming email is from a reputable source. If a domain or IP address linked in your signature has a poor reputation or is listed on a blocklist, it can cause your entire email to be rejected or sent to the spam folder. This is particularly true for real-time blocklists (RBLs) that rapidly update their data. Even if your sending domain has a stellar reputation, a single questionable link in your signature can jeopardize your deliverability. Learn more about email blocklists and their impact.
Authentication impact
SPF: If images or links are hosted on domains not explicitly authorized in your SPF record, it could lead to SPF failures or warnings.
DKIM: Some email signature management tools might alter the email body in a way that invalidates the DKIM signature, leading to DKIM failures.
DMARC: When SPF or DKIM fails, DMARC policies can instruct recipient servers to reject or quarantine your emails, meaning they won't reach the inbox.
Troubleshooting steps
To diagnose the issue, I recommend a systematic approach. First, try simplifying your signature drastically. Remove all images, links, and complex formatting, leaving only plain text. If your emails start delivering, you can then reintroduce elements one by one to identify the exact component causing the problem. This elimination diet is effective in pinpointing the problematic element, whether it's a specific link, an image, or a particular HTML tag.
If you host your own images or links, check the reputation of those domains. A quick search for domain reputation tools can help you find services that assess a domain's standing. If any of your linked domains have a poor reputation, consider hosting those assets elsewhere or using reputable URL shorteners if necessary. Also, ensure your email authentication records (SPF, DKIM, DMARC) are correctly configured for your sending domain, especially if you're using an email service provider or a third-party signature management tool. Misconfigurations here can be a primary reason why cold emails go to spam.
Finally, when an email doesn't deliver, it's essential to check if you receive a bounce message. Bounce messages provide critical clues about why an email failed, such as recipient inbox full, invalid address, or specific filtering reasons. If there's no bounce message, the email might be silently dropped by the recipient's server due to high spam scores or blocklist hits. In such cases, testing with a disposable Gmail or Outlook address can sometimes provide more detailed warnings in the inbox or spam folder about the signature content.
Email client settings also play a role in how your signature is handled. Many email clients have default settings that might interfere with how signatures are displayed or even appended to your messages. For example, some clients might not add the signature when you reply or forward an email, or they might default to plain text mode for certain message types. This can cause the signature to be ignored or appear differently than intended, impacting the overall perceived legitimacy of your email.
It's worth checking the specific settings in your email client (e.g., Outlook, Gmail, Thunderbird) to ensure your signature is correctly configured for all message types, including new emails, replies, and forwards. Look for options related to default signatures, email format (HTML vs. Plain Text), and how images are handled. Sometimes, simply verifying the email format can resolve issues where your signature isn't displaying correctly.
Recipient server filtering
Finally, the recipient's mail server or email client settings might also be at fault. Some highly restrictive mail servers are configured to block or quarantine emails containing certain types of content or links, even if your signature is perfectly legitimate. Corporate firewalls and strict spam filters can be particularly aggressive. If your emails are consistently failing to deliver to specific domains, it might be worth asking the recipient to check their spam folder or add your sending domain to their safe sender list. This often happens when marketing emails are blocked by the recipient.
The combination of content issues, authentication complexities, and client/server configurations means that troubleshooting signature-related delivery problems requires a comprehensive approach. It's often a process of elimination to identify the exact cause. However, by systematically checking these areas, you can significantly improve your email deliverability and ensure your messages, complete with your professional signature, reach their intended destination.
If you're facing persistent email deliverability challenges, remember to leverage tools and resources available to you. Regularly monitor your domain's reputation and perform routine email deliverability tests to catch potential issues before they impact your communications.
Views from the trenches
Best practices
Keep signatures concise, with only essential contact information and minimal links to high-reputation domains.
Always use optimized, small-sized images and host them on reputable content delivery networks (CDNs).
Ensure your email client sends messages in HTML format for rich signatures to prevent images from becoming attachments.
Regularly check the reputation of all domains linked in your signature to avoid issues with blacklists or spam filters.
Implement and monitor DMARC to gain visibility into email authentication failures caused by signature elements or other issues.
Common pitfalls
Including too many links or links to low-reputation websites can trigger spam filters.
Using large, unoptimized images directly embedded in the signature can increase email size and cause delivery issues.
Having misconfigured SPF, DKIM, or DMARC records can lead to authentication failures, especially when third-party signature tools are used.
Not testing your email signature across various email clients and mailbox providers (e.g., Gmail, Outlook, Yahoo).
Ignoring bounce messages, which often contain crucial information about why an email was not delivered.
Expert tips
Use a text-only signature for troubleshooting: If your emails are not delivering, try sending a plain text email with no signature. If it delivers, reintroduce signature elements one by one.
Verify image hosting: Ensure any images in your signature are hosted on a secure, reputable server. Malicious or compromised image hosts can severely impact deliverability.
Check for hidden characters or complex HTML: Sometimes, signatures copied from external sources can contain problematic HTML tags that trigger spam filters. Simplify the code if possible.
Review sender reputation: If your signature includes links to external domains, ensure those domains have good reputations themselves. A poor reputation on a linked domain can affect your sender score.
Test on different mailbox providers: What works for Gmail might not work for Outlook or other corporate mail systems. Test your signature with various recipients to catch subtle rendering or delivery differences.
Marketer view
Marketer from Email Geeks says they had a text message signature without any links or images, and emails wouldn't deliver with it, but would without it. They then found out it had a phone number and a domain in it. They were advised to remove the phone number and domain, then re-add one at a time.
2021-11-26 - Email Geeks
Marketer view
Marketer from Email Geeks says they weren't getting bounce messages, and the email wasn't in spam either.
2021-11-26 - Email Geeks
Ensuring your signature doesn't hinder delivery
Email signatures are a vital part of professional communication, but their complexity can inadvertently hinder email delivery. By understanding the potential issues related to signature content, email authentication, client settings, and recipient server behavior, you can take proactive steps to ensure your messages reach the inbox. Always prioritize simplicity and verify the reputation of any linked assets within your signature.
Regular testing and attention to bounce messages will be your best allies in maintaining high deliverability rates. Remember, a clean, compliant signature not only looks professional but also helps ensure your emails land where they belong.
Google and 
Microsoft) are constantly on the lookout for suspicious patterns that indicate spam or phishing attempts. A signature with too many links, poorly formatted HTML, or images hosted on untrustworthy domains can easily trip these filters. Even a legitimate link to a domain with a low reputation can lead to delivery issues. Complex signatures with links and images hosted in untrustworthy environments are a common problem.
