You have configured Signature 365 to apply signatures server-side but emails with large attachments are not getting a signature applied.
To ensure efficient processing of email, a limit is imposed on the size of email to which Signature 365 will add a signature server-side.
The maximum email size accepted by the Signature 365 service for server-side signatures is 30MB.
The underlying encoding of emails during transport can increase the size of an email by up to 30%, so a 24MB email (at the time of sending) may have an actual size of 30MB during transport.
Deploy the Signature 365 add-in for Outlook to apply signatures client-side and use Outlook for Windows or Outlook on the Web to send emails with large attachments.