I have solved my own problem here. The outgoing message which claimed not to have been sent (many addressees did receive their copy, but presumably the software decrees that if even one recipient fails to be sent a copy, the message is deemed to have failed, which I suppose is logical), is of course diverted to outbox. from here, an attempt to send it is made every time one connects to the net or sends any other message. This gave me the impression that all outgoing messages were afflicted with the error. Now I have deleted the offending message from Outbox, bingo, no more error message. By the way, although I didn't get any suggestions from this forum, I tried entering 'Error 501' on Google and it turns out to be quite a common problem, solutions listed at click here .
Now Norton won't liveupdate....misery ensues