An SMTP server is a server that sends mail from your account and delivers it to whomever you sent it to. If you have a domain name hosted on our server, then you can use you domain name as your SMTP server.
Example:
SMTP Server Setting: mail.yourdomainname.com
In some cases, your ISP may require that you use their SMTP server setting of your domain. Some common ISPs that require this is AT&T and Earthlink. There are many others.
If you incoming mail works fine, but your outgoing mail does not work, check your SMTP server settings in your E-mail program preferences and call your ISP to find out their protocol for SMTP mail.
Also, some ISPs require authentication. If this is the case then you need to enter your ISP settings in your E-mail program under "My Server Requires Authentication".