A signature is not applied to emails with large attachments
Why server-side signatures are skipped on emails with large attachments and the size limits involved.
If you have configured Signature 365 to apply signatures server-side but emails with large attachments are not getting a signature applied, this is because a size limit is imposed to ensure efficient email processing.
Note
The maximum email size accepted by the Signature 365 service for server-side signatures is 30 megabytes.
The underlying encoding of emails during transport can increase the size of an email by up to 30%. A 24 megabyte email, at the time of sending, may have an actual size of 30 megabytes during transport.
To apply signatures to emails with large attachments, deploy the Signature 365 add-in for Outlook and use Outlook for Windows or Outlook on the Web to send them.