An SMTP (Simple Mail Transfer Protocol) server is a type of server used for delivering emails.  When you signed up with your ISP (Internet Service Provider) they should have given you the details of their smtp server. 

The name of the server will normally be post, mail, or smtp followed by the name of the ISP.  For example, if your ISP is called bloggs then their SMTP server may be called post.bloggs.com.  If you are unable to find the details of your ISP's SMTP server try looking at their website, or give them a call. 

By convention, the SMTP port will normally be port 25, however your ISP can assign any port they wish, and if you are working behind a Firewall it is possible that you will be using a different port.  Contact your ISP / Systems administrator if in doubt. 

You can also check your SMTP settings by looking at the settings in you email package.  If you are using Microsoft (r) Outlook Express for example look at Tools -> Accounts -> Properties -> Servers -> Outgoing Mail (SMTP)