I know how frustrating that can be! There are several things that could be causing a problem besides the SMTP port, so it sometimes helps to work through each of the potential problems. To that end, and in the interests of "first things first", are you able to send, and receive, mail via webmail.yourdomain.com? If not, you email account is just not setup completely yet (it sometimes takes as much as a full day for a new email account to go "live"). If you can send and receive via webmail, then it's time to move on to other possible issues. [winkl]
The correct entry for your email server (POP3, IMAP, and SMTP) is "mail.yourdomain.tld".
You could be missing a step, though it's hard for us to tell without more information. It would help to know what operating system and email client you are using, and your ISP may also be relevant.