Confusion about email size

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There is a limit to the size of individual emails that the ServiceNow servers allow. This is currently set to 25MB for both inbound and outbound emails via ServiceNow email servers in the datacenters, this is a common and reasonable process and most customers are aware of the process as they have similar limits on their own corporate emails.
What catches out a lot of users is that when they look at the total size of an email in their email client (for example, Outlook) or in the instance, the size says it is under 25MB but will fail to send (or receive) emails and report that the email size is too big.

Common symptoms

  • After several attempts to push emails through to our instance, they were bounced back as being to large even though the size was verified email size was under 25MB.
  • The following error appears when trying to send emails from our instance but the email records shows it is on 20MB. SMTP Postfix server: “552 5.3.4 Error: message file too big “

Explanation

The limit for both inbound and outbound emails is 25MB, in total size. That means that any email needs to be _smaller_ than 25MB (Megabytes) to be allowed to pass through the email servers. All emails that are _sent_ from an instance will be encoded (64bit MIME) which increases the original size to 137% which means that any email that shows in the instance as being less than 18.5MB (25MB : 1.35) will be under the 25MB limit and will be allowed through.

It is worth noting that these figures will only ever be approximate due to the process of encoding files, every file reacts differently to the encoding (although the same file will always encode that same) so if fileA=17MB it may end up being 23MB after it is encoded whereas if fileB=17MB it may end up being 23.1MB after it is encoded.

Question: Is this limitation per email or per each attachment?

Answer: The size restriction is per email and it’s total size including all attachments and email contents.

I have included the details and calculations below to help explain the limit a bit better.

The total size email that ServiceNow allows through is: 26214400 bytes (megabytes (MB) 26.21, mebibytes* (MiB) 25.00)
*A Mebibyte is the true technical word for what most normal people refer to as a Megabyte

What we have not take into account is MIME encoding, which bloats the files up to approx. 137% of its original size.

  • 19.8 MiB – 20761804 bytes + MIME encoding = 28443671 – NOT ALLOWED – File is greater than 26214400 bytes
  • 18.5 MiB – 19398656 bytes + MIME encoding = 26576158 – NOT ALLOWED – File is greater than 26214400 bytes
  • 17.5 MiB – 18350080 bytes + MIME encoding = 25139609 – ALLOWED – File is smaller than 26214400 bytes

The data above is based on the way ServiceNow _sends_ emails and the encoding they use on their servers, in a case where we are talking about ServiceNow _receiving_ email then the encoding will be done by your companies email client/servers and may be different and increase files by different amounts. As we do not know what this process on the client side is we can only assume that a bounced email is over the limit because of this encoding.

If you have any questions about this topic, please don’t hesitate to contact me on: .img[at].img.

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