The incoming and outgoing servers are probably not the same for Comcast.
Incoming may be POP3, but does not have to be. Outgoing is SMTP.
You need to find out what the incoming and outgoing servers are, whether they both require authentication (a lot of ISP's are using authentication on thier SMTP servers), and what mail protocols they are using for inbound (POP3, IMAP...).
Just ask them Rip. Once you are armed with that information, you can setup Outlook.
mail.comcast.net is the inbound server, but it apparently is not the outbound server Rip. The error you are getting is during a send process as port 25 is the SMTP port.