Outlook Express has been used for a long time with no problems and nor
alterations have been made. 4 days ago the sent emails were rejected. I
turned off computer over night and it worked again the next day. Then,
yesterday, the sent emails were rejected and I shut off computer overnight.
Error problem still exists. Receiving emails works OK. I can send OK
through webmail at Comcast, but not from OE. Here is the Microsoft error
message:

The message could not be sent because the server rejected the sender's
e-mail address. The sender's e-mail address was '.
Subject 'testing', Account: 'Bob Wilmot', Server: 'smtp.comcast.net',
Protocol: SMTP, Server Response: '550 5.1.0 Authentication required', Port:
25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC78