When you have a large Mail e.g. 15MB and sign it, it can be sent.
But when looking at the Mail GpgOL fails to add all the attachments extracted from the signed mime structure because the signed mime structure is still part of the message but hidden and the verified attachments are now also part of the message.
So the Add invocation fails with an exception that the message size exceeds the server limit. GpgOL then receives the "AttachmentRemove" event and even brings up the attachment has been removed warning and the attachments are not all shown.
Prio high as for a user this is indistinguishable from a data loss issue.