You have configured Email Signature Manager to use the Signature Injection Service for Office 365 to apply signatures to emails sent from mobile devices, 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 Injection Service will add a signature.
The maximum email size accepted by the Signature Injection Service is 30MB.
The underlying encoding of emails during transport may add up to 30-40% to the size of your email, so a 22MB email (at the time of sending) may have an actual size of 30MB during transport.
Use Outlook for Windows to send emails with large attachments.