Email signatures can affect deliverability, but a well-built one rarely sends mail to spam by itself. Whether yours causes problems depends on how it's built and how it's deployed.

Heavy HTML, oversized images, a stack of external links, and redirect-based click tracking all add to the signals spam filters weigh. Deployment matters too, because a tool that sends mail from its own domain introduces a new sending source your existing setup has to account for. Most teams never check any of this until inbox placement drops.

This guide walks through what in an email signature actually affects deliverability, how it interacts with your SPF, DKIM, and DMARC records, how your deployment model changes the picture, and what to check first if mail starts landing in spam after you roll out a tool.

What in an email signature actually affects deliverability

Spam filters score the whole message against a mix of signals: your sending domain's reputation, whether the message passes authentication, how recipients engage with your mail, and the content and structure of the message itself. Your email signature sits in that last category. And because the same template rides on every message you send, a small flaw in it becomes a company-wide one.

Four parts of a signature matter most: its HTML, its images, its links, and how it tracks clicks.

HTML weight and code complexity

Every email signature adds HTML to your message, and both how clean that HTML is and how much of it there is affect the message.

Clean, lean markup is barely noticeable. Bloated code, thick with leftover comments, duplicated style blocks, and copy-paste formatting, adds unnecessary weight to every message you send.

The better-documented risk is an image-heavy signature: one built mostly from a single large image with little real text is a pattern spam filters have long associated with junk mail. In practice, keep the markup tidy and make sure your contact details sit in real text, not baked into an image.

How images are handled

Images are where signatures gain the most weight, and there are two ways to include them. You can embed the image directly in the message, or host it somewhere and link to it:

• Embedded images render reliably and don't depend on an external server, but they add to the size of every message.

• Linked images keep the message light, but some clients block them by default until the recipient loads them, and each one adds an external domain to your mail.

Neither option is automatically better; the right call depends on your recipients and how you weigh reliable rendering against message size. What both share is a size ceiling. Exclaimer's guide to embedded versus linked signature images  recommends keeping each image to 50KB or less, so it loads quickly and doesn't push the message toward spam filters.

Links, banners, and external domains

Every link and hosted asset in a signature points to an external domain: the site your logo opens, a campaign banner's destination, social icons, an image host.

What matters most is the reputation of those domains, since filters check links against URL blocklists. A signature linking to a few well-known domains is unremarkable. One packed with banners, badges, and shortened URLs pointing to a scatter of unfamiliar destinations starts to resemble marketing mail, which tends to draw more scrutiny.

Two habits keep this in check. Use full, branded destination URLs rather than link shorteners, which hide where a link actually goes and are a well-known spam signal. And keep the number of distinct external domains modest, especially once you start adding promotional banners on top of the standard logo and social links.

Click tracking: UTM tags versus redirect-based tracking

If you track clicks from signature links or banners, the tracking method affects how the link looks to a filter:

• UTM parameters append tracking tags to your own destination URL, so the link still points to your domain and nothing is rewritten.

• Redirect-based tracking instead replaces the link with one that routes through a tracking domain first, then forwards the recipient to the real destination.

That redirect is what lets the tool log clicks as they happen. UTM tags don't count anything on their own; they just label the visit, so the destination site needs its own analytics set up to record it. However, the redirect is also the part a filter can read as a risk, especially when the tracking domain is unfamiliar or unrelated to your brand.

Where inbox placement matters more than in-tool click reporting, UTM tags are the safer default, since they leave the link pointing at your domain. If you do rely on redirect-based tracking, point the tracking domain at your own using a CNAME record, so the link reads as yours rather than a third party's.

How email authentication fits in: SPF, DKIM, and DMARC

Your email signature doesn't set up authentication, but the way it's deployed can break it, and broken authentication is one of the most direct routes to the spam folder. Three standards decide whether a receiving server trusts your mail; mail that fails their checks is far more likely to be filtered, and can be rejected outright depending on the receiver's policy.

This foundation is often missing. In Exclaimer's State of Business Email 2025 research, fewer than a third of organizations had implemented SPF, DKIM, or DMARC, so most senders are starting from a weaker position before a signature even enters the picture. The Microsoft 365 guides linked above are the place to start if you haven't.

A signature can undermine authentication in two ways, and both come down to how it's added:

• The sending source. If your signature tool sends mail through its own servers rather than your existing mail flow, that source has to be authorized in your SPF record, or the message fails the check. More sending sources also consume SPF's limit of 10 DNS lookups; exceed it, and SPF can fail for all your mail, including messages from legitimate senders.

• DKIM. Because DKIM signs the message contents, anything that changes the message after it's signed invalidates the signature, unless the mail flow signs it again afterward. A tool that inserts a signature at the wrong point in the flow can quietly break DKIM on every message it touches.

Whether either risk applies to you comes down to how your signature tool is deployed.

How your deployment model changes the deliverability picture

Everything above applies to any signature. What decides whether those risks reach your mail is how the signature gets added, which comes down to two deployment models.

• Client-side deployment adds the signature before the message is sent, inside the mail client. Mail then leaves through Microsoft 365 as it always has, so nothing new sits in the sending path: no SPF include to add, and Microsoft signs DKIM as normal. The tradeoff is coverage. The signature only appears where the tool is installed, so mail from an unmanaged device or a phone can go out bare.

• Server-side deployment adds the signature after the message is sent, either by routing mail through the tool's own cloud service or by a transport rule inside Microsoft 365. It covers every message from every device, its main advantage, but adding the signature mid-flow changes how the message authenticates (as the following table lays out).

The SPF row is a one-time configuration step. The DKIM row is where server-side tools genuinely differ, and it's worth understanding before you pick one. DKIM is order-sensitive: Microsoft 365 signs the message as it leaves, so if a tool inserts the signature afterward, the body no longer matches what was signed and DKIM fails, while the mail still looks correct to the sender. A hand-built transport rule does exactly that. Getting the order right is the fix: Exclaimer's cloud solution, for one, removes the existing DKIM signature when the message arrives, adds the signature, and passes the message back to Microsoft 365 to sign DKIM once the signature is in place. Because the signing happens last, the content the recipient checks is the content they received, so DKIM validates.

Neither model is universally better. Client-side keeps the sending path untouched but only covers managed devices; server-side covers everything, at the cost of a DKIM ordering step both forms must get right, plus an SPF include if the tool routes mail through its own service.

Best practices for deliverability-safe email signatures

Keeping a signature deliverability-safe comes down to a handful of habits, none of which mean stripping it back to plain text.

• Keep the HTML clean and light. Use tidy markup with no leftover comments or duplicated styles, and put contact details in real text rather than baking them into one large image.

• Keep each image to 50KB or less. Choose embedded or linked images deliberately, based on how your recipients read mail and whether you need to update branding after sending.

• Use full, branded destination URLs. Skip link shorteners, which hide where a link goes and are a known spam signal.

• Go easy on banners and badges. Each one adds another external domain to every message, so keep them to what earns its place.

• Prefer UTM tags for click tracking. If you rely on redirect-based tracking instead, point the tracking domain at your own so the link reads as yours.

• Get SPF, DKIM, and DMARC in place. Mail that fails authentication is among the most likely to be filtered.

• Match deployment to authentication. If your tool routes mail through its own service, add its SPF include and mind the 10-lookup limit, and confirm DKIM is signed after the signature is added, not before.

What to check if deliverability drops after deploying a signature tool

If inbox placement drops right after you roll out a signature tool, work through these checks in order. The most common causes sit at the top.

1. Read the authentication results on a test message. Send a message through the live setup and open its Authentication-Results header, or run it through a lookup tool. SPF, DKIM, and DMARC each report pass or fail there, and that one line tells you whether the problem is authentication or something else.

2. If DKIM fails, check the signing order. A DKIM failure with a body-hash mismatch most often points to the signature being added after Microsoft 365 signs the message. Confirm the mail flow signs DKIM once the signature is already in place, not before.

3. If SPF fails, check the include and the lookup count. A tool that routes mail through its own service has to be authorized in your SPF record. Confirm its include is there, and that you haven't pushed past SPF's 10-lookup limit, which makes SPF fail for everything.

4. If authentication passes but mail still lands in spam, look at content. That points away from authentication and toward the signature itself: oversized images, a stack of tracked or shortened links, or too many unfamiliar external domains. Compare a message carrying the signature against one without it.

5. Check who's affected. If only some recipients route your mail to spam, note which. A problem isolated to one provider, say Gmail but not Outlook, points to reputation or content, since a broken SPF or DKIM record tends to hurt everywhere at once.