Outgoing email is a No-Go

Can't send email.
I can receive it just fine.

I installed McAfee Antivirus Plus 2010 about a week ago.
No problems, except this one.

When I try to send email, I get the following error:
The connection to the server has failed. Account: 'Comcast Online', Server: 'smtp.comcast.net', Protocol: SMTP, Port: 25, Secure(SSL): No, Socket Error: 10060, Error Number: 0x800CCC0E

I read a bunch of suggestions online and the only thing that made sense was to change the port.
Multiple suggestions said to change the port from 25 to 587.
I tried it and it didn't work.
After I changed it to 587, I got the following 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 'shempdevil@comcast.net'. Subject 'test', Account: 'Comcast Online', Server: 'smtp.comcast.net', Protocol: SMTP, Server Response: '550 5.1.0 Authentication required', Port: 587, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC78

Any idea what might be going on?

Have a look here and be sure your e-mail client is setup the same.

This worked 100%!!!!!!!!!!!!!!!

Thanks for your time, I appreciate it.

Glad to help

Happy computing