Email Signature Deliverability Best Practices: 10 Rules to Keep Your Emails Out of Spam
Your email signature can hurt your deliverability in ways most people don’t expect. Large images, too many links, messy HTML, and certain hosting choices all contribute to spam scores. This guide covers all of it — with specific numbers, real examples, and a checklist you can use right now.
By the NeatStamp Team · Updated March 2026 · 15 min read
How signatures affect deliverability
Email deliverability is determined by spam filters evaluating hundreds of signals. Your email signature contributes to several of those signals simultaneously — email body size, link count, link domain reputation, image hosting method, and HTML complexity.
The spam scoring model
Spam filters like SpamAssassin assign weighted point values to signals. No single signal usually tips the balance — emails are scored cumulatively. A signature with one or two issues might not cause problems. A signature with five issues on top of a marketing-heavy email body can.
The key insight is that your signature represents a consistent element across every email you send. If your signature has a deliverability problem, that problem affects 100% of your outbound emails — not just one campaign.
The four ways signatures hurt deliverability
File size inflation
HighBase64-encoded images bloat the email HTML. Large emails (over 100KB) trigger spam signals and are also clipped by Gmail. A single 20KB image encoded as base64 adds approximately 27KB of HTML text.
Link count and link domain diversity
Medium–HighEach link in your signature is an external reference that spam filters evaluate. Multiple links to different domains — website, LinkedIn, Twitter, Instagram, Calendly, portfolio site — multiply the number of domain reputation checks. Any one of those domains having reputation issues can affect your score.
HTML complexity
MediumNested tables, CSS style blocks, non-standard HTML attributes, and inline JavaScript (never do this in a signature) all contribute to complexity that spam filters flag. Clean, minimal HTML is consistently better for deliverability.
Spam trigger words
Low–MediumCertain words and phrases in the text of your signature can contribute to spam scores — particularly promotional language, financial terms, or excessive capitalisation. These are more often a problem with signature banners and promotional CTAs than with the contact information section.
The 10 rules for deliverability-safe signatures
These rules come from spam filter documentation, email deliverability research, and real-world testing. Follow all 10 and your signature will be essentially neutral from a deliverability standpoint.
Never embed images as base64
Host all images externally and reference them by URL. Base64 encoding inflates file size, looks suspicious to spam filters, and is stripped by several email clients anyway. Use a CDN or your company website to host your logo and headshot.
File size impact: a 20KB image becomes ~27KB of base64 text
Keep total HTML under 15KB
The HTML code of your signature — excluding the content of externally hosted images — should be under 15KB. Under 10KB is better. Measure this by saving your signature HTML to a .txt file and checking the file size.
Gmail clips entire emails above 102KB total. Signatures contribute.
Use a maximum of 5 links
Every link is a potential spam signal. Choose your links carefully: website, one social profile (LinkedIn is the safest choice), phone (tel: link), email (mailto: link), and one optional CTA. Cut everything else. Three links is ideal.
Each additional link beyond 5 incrementally increases your SpamAssassin score
Keep all links on the same domain or well-known platforms
Links to your company website and to established platforms (LinkedIn, GitHub) are low-risk. Links to unusual domains, URL shorteners, or newly registered domains can hurt your score. Never use bit.ly or similar shorteners — spam filters distrust them.
URL shorteners can add 0.5–1.0 SpamAssassin points
Host images on a reputable CDN
Your image hosting domain affects your deliverability. Images hosted on a domain with low reputation or one that's been used for spam can hurt your score. NeatStamp's CDN is specifically maintained for email signature image hosting with a clean sending reputation.
Your own company domain or an established CDN (NeatStamp, AWS, Cloudflare) are the safest choices
Optimise image file sizes
Large images load slowly and are disproportionate to the context of an email signature. Logo: under 20KB. Headshot: under 25KB. Banner: under 40KB. Use PNG for graphics with transparency or text, JPG for photographs. WebP has limited email client support and should be avoided.
Total signature images: aim for under 60KB combined
Set explicit image dimensions in HTML attributes
Use both width and height HTML attributes on every image element — not just CSS. Without them, Outlook renders images at full size, which can make a 2x retina image display at 2–4x its intended size. Explicit dimensions prevent layout-breaking surprises.
Example: <img src='...' width='150' height='50' style='width:150px;height:50px;'>
Avoid promotional language in your signature
Words and phrases that trigger spam filters are more likely to appear in signature banners than in contact info. Avoid: FREE, guaranteed, limited offer, act now, click here, no cost, earn money, buy now. Your CTA link text should be descriptive and specific, not generic ('Book a call' not 'CLICK HERE FOR FREE CONSULTATION!!!').
Exclamation marks in signatures can add 0.1–0.3 SpamAssassin points each
Use clean, table-based HTML
Email HTML should use table-based layouts with inline styles. Avoid CSS class attributes (many email clients ignore them), JavaScript (never use it), CSS hover states, media queries in the signature HTML (use only for the enclosing email template), and non-standard HTML5 elements. The simpler the HTML, the better.
Spam filters assign higher trust to simple, well-formed HTML
Test before deploying to the team
Run your signature through a deliverability checker before rolling it out. Send test emails through mail-tester.com or use NeatStamp's built-in deliverability checker. Check your spam score, image rendering, and link validity. A 10-minute test before deployment prevents issues across hundreds of emails.
Target: a mail-tester.com score of 9/10 or higher
Image rules in detail
Images are the most common source of deliverability problems in email signatures. Here are the specific numbers to aim for.
| Image type | Display size | Actual file size | Format |
|---|---|---|---|
| Logo | 150–200px wide | Under 20KB | PNG (transparent) |
| Headshot | 80–100px square | Under 25KB | JPG |
| Banner / CTA | Max 600px wide | Under 40KB | JPG or PNG |
| Social icon | 20–24px square | Under 3KB each | PNG or SVG |
Retina (2x) images without file size problems
Retina screens display images at 2x density. A logo displayed at 150px wide needs a 300px source image to look crisp on retina. But a 300px PNG can get large fast.
The trick: export your logo at 2x dimensions (300px wide for a 150px display size), but compress it aggressively with a tool like Squoosh, TinyPNG, or ImageOptim. A 300px PNG logo can be kept under 12KB with proper compression — it looks sharp on retina and doesn’t bloat your email.
Then set the HTML attributes to display it at the intended size:
width="150" height="50"
style="width:150px;height:50px;display:block;"
alt="Company Name" />
The actual file is 300×100px, the display size is 150×50px. Retina screens display it at full sharpness; standard screens display it correctly scaled.
Link rules in detail
Links are the second most common deliverability issue in email signatures. Here’s exactly how to handle them.
The safe links in a signature
- Website — your company homepage or a dedicated landing page
- Phone — tel:+44... links (these are local calls, don't hit external domain checks)
- Email — mailto: links (same, no external domain)
- LinkedIn — high-reputation domain, routinely trusted by spam filters
- One CTA — a specific page on your own website is the safest choice
Links to avoid
- URL shorteners (bit.ly, tinyurl.com) — often flagged because spam abuses them
- Newly registered domains (under 6 months old) — lower reputation
- Links to domains with known spam history — run a check at mxtoolbox.com if unsure
- Instagram, TikTok, Facebook — lower relevance in business contexts and add link count risk
- Calendly, Cal.com, Acuity — fine individually, but if you already have 3 links, this is #4+
How to track link clicks without hurting deliverability
If you want to measure which links in your signature get clicked, avoid using UTM redirect services that route through a tracking domain. Instead, use UTM parameters directly appended to your own domain URL:
https://track.signaturetool.com/click?url=https://yoursite.com&id=abc123
<!-- Use instead: -->
https://yoursite.com/contact?utm_source=email&utm_medium=signature&utm_campaign=sales
The second approach keeps all links on your own domain, which spam filters trust more, and Google Analytics (or your analytics tool) still tracks the click.
HTML weight and file size
The HTML of your signature — the actual code, not the images — has a direct impact on deliverability and user experience.
Why HTML size matters
Gmail clips email messages that exceed 102KB of HTML. The clipping shows a “[Message clipped] View entire message” link at the bottom of the email. When an email is clipped, your signature is usually the first casualty — it appears after the body content, so it gets cut.
Your signature HTML contributes to the 102KB limit on every email. A 25KB signature HTML leaves only 77KB for your email body. In a long reply thread, each reply appends the previous emails — making clipping increasingly likely.
What inflates signature HTML size
How to check your signature HTML size
Copy your signature HTML and paste it into a text file. Check the file size in your file system. On Mac: right-click → Get Info. On Windows: right-click → Properties. Aim for under 15KB. If you’re over 20KB, identify the largest elements and optimise them first.
NeatStamp’s signatures are optimised to stay under 12KB for the HTML. The deliverability checker measures this automatically.
Spam trigger words in signatures
Spam trigger words in email body content are well documented. They apply equally to signature text. The risk is highest in promotional banners and CTA buttons added below the contact information.
High-risk words and phrases
Better alternatives for common signature CTAs
Formatting that triggers spam filters
- ALL CAPS text — spam filters flag this aggressively
- Excessive exclamation marks!!! — each one adds a small penalty
- Dollar signs in quantity — $$$, $FREE$, or repeated $ symbols
- Coloured text on coloured background (especially if the contrast is low — looks like hidden text tricks)
- Multiple font sizes in rapid succession — associated with visual spam tactics
How to test your signature’s deliverability impact
Testing before deployment is the most reliable way to catch problems. Here are the tools and methods that actually work.
mail-tester.com
Mail Tester is a free tool that gives you a unique test email address. Send an email from your account using your signature, and it analyses the email and gives you a spam score out of 10. It checks SPF, DKIM, DMARC, content triggers, and link reputation. Aim for 9/10 or higher. Free users get 3 tests per day.
Important: when testing your signature specifically, write a short, neutral email body (not promotional text) so the body doesn’t influence the result. The goal is to isolate the signature’s contribution.
GlockApps
GlockApps tests deliverability against multiple inbox providers simultaneously — Gmail, Outlook, Yahoo, AOL, and others. It shows you whether your email lands in inbox, spam, or promotions tab across each provider. This is more useful for marketing campaigns but can reveal if your signature is causing inbox placement issues with specific providers.
MXToolbox blacklist check
If your signature includes links to your company website, run your domain through MXToolbox’s blacklist checker to confirm it’s not on any major spam blacklists. This is especially worth doing if you’ve recently moved to a new domain or if your domain was previously used by someone else.
Send to your own accounts first
Before rolling out a new signature to your team, send test emails to accounts on Gmail, Outlook.com, and Yahoo Mail from your work account. Check:
- Does the email land in inbox or spam?
- Does the signature render correctly in each client?
- Are images loading?
- Are all links working?
- Does the email look right on mobile?
NeatStamp’s deliverability checker
NeatStamp includes a built-in deliverability checker for signatures built in the editor. Here’s what it checks and how to use it.
The checker runs automatically when you build your signature in the NeatStamp editor. Each check passes or fails, and failing checks include specific fix instructions. It’s also available as a standalone tool on the deliverability page if you want to check a signature built elsewhere.
The checker doesn’t replace sending a real test email — it analyzes the signature HTML in isolation. Use it as a first pass before testing with mail-tester.com.
Final deliverability checklist
Run through this before deploying any email signature.
Images
- No base64-encoded images — all images hosted externally
- All images have explicit width and height HTML attributes
- Logo under 20KB, headshot under 25KB, banner under 40KB
- Images hosted on company domain or reputable CDN
Links
- Maximum 5 external links
- No URL shorteners
- No links to recently registered or unusual domains
- All linked domains checked against blacklists
HTML
- Total HTML under 15KB
- No JavaScript in signature code
- No CSS class attributes — only inline styles
- Clean, valid HTML — no unclosed tags
Text content
- No all-caps text
- No excessive exclamation marks
- No spam trigger words in CTA text or banners
- Disclaimer text is concise (under 3 lines)
Testing
- Run mail-tester.com check — score 9/10 or above
- Sent test email to Gmail — lands in inbox
- Sent test email to Outlook.com — lands in inbox
- Checked rendering on mobile
Frequently asked questions
Can my email signature cause my emails to land in spam?
Yes, it can contribute to that outcome. A signature with too many links, large embedded images, excessive HTML, certain banned words, or links to low-reputation domains can push your spam score high enough to trigger filters. No single element usually causes this alone — it's typically a combination of factors. The most common culprit is either base64-encoded images (which massively inflate email file size) or multiple external links with no text context.
How many links in an email signature is too many?
Spam filters don't have a hard cutoff, but best practice is 3–5 links maximum in a signature. That might include: website, LinkedIn, phone (tel: link), and an optional CTA. Beyond that, each additional link incrementally increases your spam score, especially if those links point to different domains.
Should I use base64 images or hosted images in my email signature?
Hosted images, always. Base64 encoding inflates image size by approximately 33%, bloating email file sizes significantly. Large file sizes (above 100KB total) reliably increase spam scores. Additionally, many spam filters specifically flag high base64 content as a signal of image-heavy spam. Use externally hosted images referenced by URL instead.
What is the ideal total HTML size for an email signature?
Aim for under 15KB of HTML (just the signature HTML, not counting externally loaded images). Under 10KB is better. Gmail clips emails over 102KB total — if your signature HTML contributes significantly to that, long email threads may get clipped, hiding your signature entirely.
Does NeatStamp's deliverability checker scan my live emails or just the signature HTML?
NeatStamp's deliverability checker analyzes your signature HTML directly — it checks image hosting method, file size, link count, domain reputation indicators, HTML weight, and known spam trigger words. It doesn't send test emails or require access to your email account. The check runs in seconds and gives you a score with specific recommendations.
Related guides
Build a deliverability-safe signature
NeatStamp generates clean, lightweight HTML that follows every rule in this guide. Built-in deliverability checker included — free.