[Crossposted to OE General, assuming OE not OL]
Post the complete (verbatim) error message.
Example:
An unknown error has occurred. Subject 't', Account: 'Verizon MyAccount',
Server: 'outgoing.verizon.net', Protocol: SMTP, Server Response: '550 5.7.1
The message you attempted to send was determined to be spam. Please visit
http://www.verizon.net/spamfaq for more information.', Port: 25,
Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC69