File Size in Email

How does email file size affect deliverability?


Spam filters, blocklists, sender reputation, trigger words… all those phrases run through email marketers’ minds when we think about email deliverability. While those are all critical factors that will impact deliverability, one aspect that is often overlooked is the email file size. The size of your email plays a significant role in whether your message makes it to the inbox and how it looks once it gets there.

There are many different (and contradictory) recommendations floating around the web for what the max is for email file size before it negatively effects your inbox placement. That’s why we dug up current file size recommendations circulating around the web, then did some testing of our own to find the “perfect” weight for your email.

At what point does email file size cause deliverability issues?

Reports vary when it comes to email file size and deliverability. The V12 group says the overall file size of the email should not exceed 50 -70KB while Responsys recommends total email size be limited to 125KB (and that’s just a few of the recommendations we found).

We created text-only, HTML emails in various sizes, from 15-650KB.

By leveraging our spam reporting, which tests against 23 of the most popular spam filters, we found that sending a file size between 15KB-100KB is A-OK. These emails successfully passed through all our spam filters with flying colors!

Deliverability issues began to occur once the email file size was over 100KB. Every email from 110KB to 650KB wound up failing multiple spam filters. Interestingly enough, once the email file size was over 100KB, the number of spam filters that failed each email stayed the same. For example, an email that was 110KB got caught in the same 7 spam filters as an email with a file size of 650KB.

Below are the spam filters that failed due to email file size:

Failed spam filters

CheetahMail ran tests on file size and deliverability as well and found identical results! Our conclusion was validated by their study as they found that a file size of 100KB will make it to the inbox if you are sending to Gmail, Yahoo!, AOL or Outlook. It’s better to be safe than sorry, so keep it under 100KB peeps!

Image size and deliverability

From this research, we stumbled upon another query: “Does the size (in terms of KB) of images have the same effect as the email file size?” Some resources warned images that exceeded 20KB will get flagged as spam while others said images that are under 50KB are good to go. To test this theory we created 4 emails with 16, 84, 146 and 696KB images to see what effect it had on deliverability.

We ensured the images were hosted on our site to avoid IP reputation problems on other servers. We found that image file size did not have any impact on deliverability. Based on our test results one should be more concerned with the size of the email vs. the size of the images.

However, it is important to note that using overly large emails without any supporting text in your email can raise a red flag for spam filters. This is because spammers usually display information in large images instead of text because the filter programs cannot “read” the content. If you stick with a 60/40 text to image ratio, you should avoid having deliverability issues.

Email “Clipping” in Gmail and iOS

Deliverability isn’t the only issue that can occur from email file size. Sending extra-large emails can cause your full message to not appear in its intended glory. For example, the Gmail app for iOS has a nasty habit of “clipping” emails, or cutting their content short and adding a button that says “Download entire message.” We have also seen this on Gmail for Android. This could potentially hide an email’s main content or call to action, but really any extra step a reader has to take is a huge stumbling block to the success of an email campaign. We’ve created a “cheat sheet” on how to avoid this issue, so be sure to check it out.

Gmail email clipping

Email load time

On top of email clipping, large emails also affect load time. Emails with a slow load time are more likely to get deleted than light, small emails that display in a jiffy. Your readers are inundated with hundreds of emails a day so the chances that they’ll sit patiently and watch the rainbow wheel load is unlikely.

It is also important to note that emails with a large file size load even slower on mobile than on web or desktop. Not to mention, larger images will eat up valuable megabytes in limited data plans. With about 20-50% reading email on mobile, we carefully design emails that will accommodate the limited data plans, slow networks, and various behaviors of the mobile email clients.

How to get in under the wire

If your email file size is over 100KB or skirting the line of surpassing 100KB, save a few bytes with the following tips:

  • Remove any unnecessary spaces, carriage returns or comments.
  • Remove redundant or unnecessary styles
  • Make a landing page so all of your text and images don’t have to all fit in the email!

How to avoid deliverability issues before they happen

You shouldn’t waste your time crafting a perfect email if it’s never going to end up in your subscriber’s inbox. Verify your emails are reaching the inbox with our spam testing software so you never toil away on emails that will get tangled in spam filters.

What issues have you seen regarding email file size and deliverability? Have you done testing on spam filters and seen different results? We’d love to for you to comment below regarding the quirks you have encountered in the wild or the workarounds you’ve found along the way.

13 thoughts on “How does email file size affect deliverability?”

  1. Dear,

    Thanks for this article, and test.

    I have one question. When you talk about 60/40 text to image ratio, are you talking surface?

    I mean, if we have an image of 400x600px do we must have a 600x600px full text block?

  2. Hey there,

    Funny you should ask about text to image ratio as we are currently testing this hypothesis and its effect on deliverability.

    At the point in which we wrote the blog, we had not tested whether the SPAM filters were testing the actual surface of image to text block. We are currently gathering and testing text to image ratio and we’ll have the answers for you very soon 🙂

  3. Hi Mallory,

    When talking about email size, does this consider only the email body (HTML and plaintext) and NOT attachments or does any attachment also count towards that total email size?


  4. Hi Alex,

    When talking about file size, we did not test how attachments affected file size and deliverability. My best guess, though, would be that adding an attachment would up the file size significantly.

    For reference, it is not best practice to send email campaigns with attachments. However, since we have never tested it first hand but only heard this recommendation from fellow marketers, maybe our next blog will be testing that 🙂

  5. Hi Mallory,

    Attachments are quite common, especially in the eBilling space. Why do you feel it is not best practice to send attachments?


  6. Hi Alex,

    If you check out one of MailChimp’s blogs on Host Files, they give three solid reason to avoid attachments. Check em’ out below:

    – -Many ISPs and end-users block emails above a certain size. Attachments can increase the size of your email, raising the risk of your email getting blocked (This is supported by our research above).

    – Most content and virus-scanning programs see attachments sent through any bulk mailing service as a potential security threat, and either block the attachment or the email entirely. So a majority of attachments may not reach your subscribers.

    – Sending a copy of your file to each recipient would take a lot of bandwidth, which can get pretty expensive. Hosting files within your ESP instead of allowing attachments helps delivery rates and keeps costs down. Plus, you’ll be able to track which recipients click the link for your file, and how many times opposed to an attachment.

    Alex, I do believe we are going to test this for ourselves, though, so we can know once and for all.

  7. ussualy spamers use email with big files, since they upload pictures, and ads, so that can indicate if a mail is apam or not.

  8. I appreciate that you not only did a great job on writing an astute article but backed it up with in-house testing to bring real life findings to light for others to benefit from.

  9. Hey Mallory Mongeon That was nice information. But i always use imgae ratio to 80% that will good for all email client and for gmail you have to use as much as less nested table which saves lines of code and weight which matter most in gmail and yahoo for clipping issue.

  10. I particularly use very little text and images but a while ago and resentado me emails detected as spam

  11. There is definately a great deal to learn about
    this issue. I like all the points you have made.

  12. Hello – I am very interested in Email on Acid. I read that I will be able to host images through this (AWESOME) but, is there a certain image size limit i need to stay in? For example, the file size limit is 200kb for the program I currently host images through. Thank you!!

  13. Great post, thank you! I wondered if the guidelines you and CheetahMail provide for email weight under 100kb still holds true today as the internet is faster, people have faster connection, etc. Would you still recommend this size in mid 2017?

    Thank you!

Comments are closed.