In these few days, there was strange behaviour from OE 6.00.3790.0 on
Windows 2003 (with ServerProtect running). Mail messages with myself (same
email address as my account's email address) copied (no matter CC or BCC)
failed to deliver.

--- QUOTE
Your server has unexpectedly terminated the connection. Possible causes for
this include server problems, network problems, or a long period of
inactivity. Subject 'MySubject'. Account : 'MyAccount', Server:
'smtp.myisp.com', Protocol: SMTP, Port: 25, Secure(SSL): No, Socket

Error: 10053, Error Number: 0x800CCC0F.
--- QUOTE

It was noted that the SMTP negotiation was stopped at OE side after SMTP
server's accepting my email address in "RCPT TO". It was apparent that OE6
client did not react since then, thus causing delivery problem.

*** Thunderbird running at the same time could work.

What's wrong? Is it due to windows update, because the OS was recently
updated. Or, is the server infected?