Bruce,
Today a user got the 18500 code when trying to send a 15MB attachment. The message was moved to the drafts folder and followed with a series of errors: "Could not synchronize record: FW: *EMAIL SUBJECT* to Exchange server: *ACCOUNT NAME*" In the future I have directed users to try sending with the OWA.
There was also another error that popped up today. The account was disconnected from exchange and prompted the user to reenter credentials. "HTTP Error. The server is temporarily unable to fulfill the request." Error code -18497
Any ideas?