Really strange indications.  First thing I would do in use File | Review queued mail and verify that that large message is not still in the queue.

I sent a message two days ago with too large an attachment, to it locked up the system temporarily.  Since then, if I attempt to add an attachment to an outgoing message or open an in-coming message with an attachment, the program shuts down.

 I back up with Acronis every night.  When I restored the program to the day before the time I sent too large an attachment, the program still shuts down when dealing with attachments.

Does anyone have any suggestions?