Do attachments affect deliverability?

If you're thinking about dropping a PDF into your next marketing email, pause for a second. Attachments are one of the fastest ways to watch your deliverability take a hit, and it's worth understanding exactly why before you hit send.

Spam filters treat attachments as suspicious by default. The reason is simple: malware, phishing files, and ransomware have historically traveled as email attachments. So when your newsletter arrives with a file attached, security systems don't see a helpful resource. They see a potential threat that needs inspecting.

Here's what actually happens behind the scenes when you send an email with an attachment:

• The spam score goes up. Most filtering systems assign points for risky signals. An attachment on a bulk email is a well-known spam pattern, so it adds to that score right away.
• Security gateways scan the file. This adds processing time and, for certain risky file types, can result in the email being quarantined or rejected outright.
• Executable and script files get blocked entirely. .exe, .bat, .js, and similar formats are almost always stripped or blocked by receiving mail servers before your reader ever sees the message.
• Password-protected archives raise red flags. If a scanner can't open a .zip file to inspect it, many systems default to blocking it. The reasoning is that hiding the contents looks like evasion.
• Large file sizes cause their own problems. Beyond spam scoring, oversized messages can hit server limits, causing delays or rejection.

The practical gap between transactional and marketing email matters here. A receipt PDF attached to an order confirmation has context. The recipient placed an order, they expect a document, and that expected pattern works in your favor. A PDF attached to a newsletter has no such context, especially for new subscribers, which is exactly what spam filters are trained to catch.

The better move for marketing email is almost always to host the file somewhere (Google Drive, Dropbox, your own site) and link to it from the email body. That way the reader navigates to the file intentionally, security gateways don't need to inspect your message payload, and your email stays lean.

If your use case genuinely requires an attachment, plain PDFs and standard images (JPG, PNG) are the lowest-risk options. Keep the file size small, make sure the content inside doesn't itself look spammy, and avoid anything that resembles a macro-enabled document. Even then, it's worth testing your sending score before you roll it out to your whole list.

Not sure how your emails are scoring right now? You can analyze your headers and check for filter signals with our free Email Header Analyzer. Or if this is already causing deliverability problems, our SOS hotline is free and we'll actually help you figure out what's going on.