Ask a Nerd: Does attaching files to your newsletter hurt deliverability?

This article is part of a series that answers newsletter questions we often hear from users. Send us any questions you'd like us to answer!

The body of an email is too simple, too standardized to be a vehicle for measurable harm. So the malicious and spammers use attachments instead. That is why, no matter how well intentioned your own attachments may be, spam filters and email service providers (ESPs) distrust attachments by default, dinging your deliverability if you don’t send them carefully and with discretion.

It’s a real shame, considering the conveniences newsletter attachments afford. A downloaded email accompaniment is available offline, easy to reshare, and more design-friendly than an email. They are perfect for authors who send subscribers book excerpts, nonprofits that share donor impact reports, bands that release previews of upcoming albums, and countless others. There is another way, however.

“Switch to using a hosted link instead of PDFs. We…saw a 40% improvement in deliverability after moving reports to a password-protected page,” a user wrote in a response to an /r/Emailmarketing post. “Bonus: you can track who actually opens the reports.”

A download link is virtually indistinguishable from an attachment. Clicking a link instead of an attachment yields the same results. There are advantages, too: File hosting services can set link expiration dates, share larger files than attachments allow, and let mobile users stream media instead of an awkward download.

If you absolutely must attach a file to your newsletter, however, there are a few simple rules and safeguards to minimize harm to your deliverability:

• Don’t send more than one or two files
• Don’t send attachments with every newsletter
• Total file size across one or multiple attachments should be under 2mb
• Stick to JPGs, PNGs, PDFs, and CSVs
• Describe and justify the attachment in the body of your email
• Give files clear and sensible file names
• Test deliverability with services like GlockApps or Mail-tester

You shouldn’t need to know more than that to maintain a good reputation. But, hey, a little extra detail and nuance never hurts! 

Number and frequency of attachments

To a certain degree, deliverability is determined by how much a newsletter resembles those from senders with high engagement rates, few bounced emails, and low numbers of complaints from subscribers. In other words, sending more attachments, more often, than a typical newsletter looks less like occasional resource-sharing and more like systematic file distribution. Creators and companies email files every once and a while; spammers and scammers send them all the time. 

Imagine someone you follow sends you fifty newsletters without a single attachment, then suddenly starts including multiple PDFs in every email. That’s pretty suspicious! It’s also a spike in the burden on ESPs that scan every attachment, which they do not look kindly on. 

Sending one or two files, one time, should be OK. If you’ll need to go beyond that, link to hosted files elsewhere, especially if they are large or unwelcome file types.

File types and sizes

Emailing a friend or two an attachment via Gmail? You’re limited to 25mb. In Outlook, it’s 20mb. As soon as you’re sending to a list, that ceiling comes way down. Newsletter platforms like Buttondown limit you to 2mb while recommending that you aim for under 1mb. This is more for speed reasons than security as malware doesn’t require large file sizes, and sending a 20mb file to 1-2 people is orders of magnitude easier for ESPs than sending a 1mb file to 500 people. 

Whatever the case, most audio and video files will fly past that threshold, making newsletter integrations like Bandcamp, Soundcloud, YouTube, and others the preferable option. Even files that stay under the limit can throw up red flags, as a user wrote in /r/emailmarketing. “Sending PDFs as attachments, especially in bulk, is a fast track to spam, even with SPF/DKIM set up. The file size, content type, and attachment headers can all trigger filters.”

In fact, anything other than basic documents and media will be flat out rejected. No respectable platform allows sending any type of file that can run as self-contained software (e.g. an “executable”), such as .exe, .bat, or .vbs. 

While Buttondown and others do allow attaching .zip files, they, along with .rar attachments are problematic on account of their ability to obfuscate what is actually attached. Similarly, Microsoft Office documents with macros enabled (e.g. .docm or .xlsm) open up possibilities for severe harm. 

Most newsletter platforms make it hard to unintentionally nuke your own deliverability by including safety checks such as those on attachment attributes. Still, a good rule of thumb is this: If you can’t remember anyone ever sending you the size or type of file you are thinking of sending others, don’t send it.

Attachment accessibility and testing

Subscriber behavior is one of the pillars of any deliverability score. A sender can attach a single file of an appropriate size and type but still receive a boatload of negative signals, such as subpar open rates, high unsubscription rates, or spam complaints. That’s why it’s so important to describe an attachment and why it’s necessary in the body of the email. And, for people who miss or skip the explanation, double check that the file’s name indicates what it is.

Attachments also need to be legible across platforms, on Windows or Mac, desktop or mobile, to avoid being flagged. Basic accessibility principles apply here as well, with documents that have high color contrast ratios, images that have descriptive text, and layouts that are easy to scan left to right, top to bottom. The same goes for assistive technology, with fewer deliverability risks when attachments work with screen readers and magnifiers.

When a newsletter and its accompanying attachment(s) are finalized, deliverability testing platforms allow you to send a draft to one of their addresses and see if everything comes through correctly. If it does, there’s little else you can do to ensure that emails are delivered to subscribers’ inboxes instead of spam folders.

Simple, minimalist emails win

It’s hard to imagine email without attachments (or the non-english characters they enabled). But newsletters don’t always feel like email. They have their own conventions and norms. Sure, you can attach a directory to a weekly church newsletter or a PDF as part of an email course, but what subscribers often care about the most is your writing. And for that, the body of an email is plenty.

Still, if you want to attach a file to your newsletter or embed way too many images, you don’t have to worry about what’s happening behind the scenes. “Once a week, I pull up Buttondown, write directly in the CMS UI like a heathen, and hit send, and then I don't think about it again for six days,” says Jenny Zhang of Show Up Toronto. “It just gets out of my way.”

Photo by Mika Baumeister on Unsplash