Size limit prevents emails from being sent or received
Individual email size is limited when transmitted through the ServiceNow email servers. The allowable size of an email sent is 25MBs (megabytes), for both inbound and outbound emails. In some cases, the total size of an email in an email client or an instance will appear to be under 25 MBs, but the email client will fail to send (inbound to ServiceNow) or the instance will fail to send (outbound from ServiceNow) emails and report that the email size is too large.
Common symptoms include:
- After several attempts to push emails through to the instance, they are bounced back for being too large, even though the email size appears to be under 25MB.
- The following error appears when trying to send emails from the instance: SMTP Postfix server: 552 5.3.4 Error: message file too big.
If using your own separate email infrastructure instead of ServiceNow's, the same issue can exist. However, the size limit for inbound and outbound may be different, as well as the error code reported. Consult the email administrator for your system.
All emails sent to or from an instance follow the MIME specification (Multipurpose Internet Mail Extensions (MIME)). Some data in an email (such as an attachment or an inline image) cannot be sent it is original format. The MIME specification requires this data to be specially encoded, which might increase the original size of the attachment data by approximately 137%.
This means an email in the instance must be less than 18.5MB total (25MB - 135%) to be under the 25MB limit and be allowed through. These figures are approximate, due to the process of encoding files. Each file reacts differently to the encoding (although the same file will always encode the same way). For example, if file A is17MB, it may end up being 23MB after it is encoded, whereas file B, at 17MB, may end up being 23.1MB.
Note: The size limit is per email and references the total size, including all attachments and email contents (headers, HTML body, text body, attachments, and some negligible overhead). This makes predicting the exact total size of an email difficult for the user who is sending an email, and only looking at attachment sizes in their file system, or by looking at the size in the sys_attachment table.
Refer to the following details and calculations to clearly understand how this limit works:
- The total email size that is allowed through is 26214400 bytes (26.21 megabytes (MB) or 25.00 mebibytes* (MiB)).
- The MIME encoding then increases the file size to approximately 137% of its original size. Examples of file sizes that would/would not be allowed after encoding include the following:
- 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
When your instance sends emails, this data is based on the way ServiceNow sends emails and the encoding used on ServiceNow servers. When receiving emails, the encoding is done by your company's email client/servers, which may be different and increase files by different amounts.
*A mebibyte is the technical word for what most users refer to as a megabyte.