Good sleuthing Jack! And thanks for posting your answer for us!
Problem now solved.
I use AVG for all my server and pc anti-virus, and the latest Automatic update from AVG caused some files to be incorrect.
The SBS 2003 Blog pointed me to a fix at the AVG support website, called “fixfile.exe” which I downloaded and ran, I then restarted the server, but the fault still existed, BUT the AVG website did say if this happened to run the AVG install again and select the repair option. This time it was Ok.
I then tested by sending some e-mails and checked to see they arrived and all was OK.
So I think this problem is now closed.
Thanks for trying to help.