Not a request for help, more of a heads-up as Ive solved the issue, but this might help someone else.
Late last night I went to send an email but found I quickly got a send error (0x800ccc0f) with a msg 'connection to server interrupted'. the mail stayed in the outbox and would not send. Left it until this morning and played about, tried sending the email again through 3 different accounts - and from 2 email clients. all blocked. Email diagnostics showed that the clients were talking to the email servers, but the mail was being blocked at my end.
Incoming emails received no probs, net and Webmail access also not affected.
The original email had 2 attachments on it totalling 6mb and i wondered if this was a factor - even basic plain text with no attachments was blocked.
Anyway I turned OFF the outgoing mailscan option and hey presto, normal service is resumed. I have reactivated it and again sending is blocked. This leads me to think that a recent update by either AVG or MS is the cause.
My kit is XP Home, SP3. email clients Outlook 2003 / OE6. I dont turn off the PC as a rule, just sleep it, although it was restarted yesterday. ISP is Virgin/NTL. I hadnt installed or removed any software yesterday, but MS sent out updates earlier this week, so it could be that or an AVG update in response any time within the last 2 or 3 days thats caused it.