WDTBD many thanks for your reply, I had searched PPrune & Microsoft but didn't find this article. That certainly sounds like my problem, however...
I'm not logged on to a LAN, I'm not trying to send through another ISP's SMTP gateway, I am using ADSL but not to get to another ISP, my ISP does not block all SMTP Relay traffic, I have a static IP address, the error message isn't random (it's always the same). I tried the resolution mentioned (select My outgoing server (SMTP) requires authentication) but it didn't help.
Problem "solved" by going back to Outlook Express, which still works fine, guess I'll live without Calendar.
Thanks again
-ev-