Can't send e-mail messages in Outlook

I'm using Windows XP and MS Outlook 2003. I can receive e-mails, but I
cannot send e-mails. When I send an e-mail, it is moved out of the Outbox
and placed in the Sent folder - appearing as though it was sent - but it
really wasn't. Within seconds, I receive the following message.

Task 'freespirit100 - Sending' reported error (0x800CCC0F) : 'The connection
to the server was interrupted. If this problem continues, contact your
server administrator or Internet service provider (ISP). The server
responded: 553 From: address not verified; see
http://help.yahoo.com/l/us/yahoo/mail/original/manage/sendfrom-07.html'

I contacted my ISP (Verizon), but they said their servers were up and
running. They were correct because I was able to successfully send e-mails
from the primary e-mail account (thru Verizon). They verified that the POP3
server and SMTP server addresses were correct. They also verified that the
e-mail address was correct.

I turned off Norton Firewall and Antivirus, rebuilt the account, changed the
password, ended numerous programs that may have been using up much-needed
resources, then tried to send a message. Still, I received the same error
message as reflected above.

Any suggestions would be greatly appreciated.

Thanks Mike.