SMTP: acronymic for Simple Mail Transportation Protocol. This is the protocol used to send email from server to server across the internet, as opposed to the various protocols used by end-users to retrieve their mail from those servers, like POP3 and IMAP.
It is possible (and useful) to troubleshoot SMTP servers using telnet - see Daemons, testing via Telnet for a simple example.
FreeBSD can be used with many different SMTP server applications, such as sendmail or qmail. The maintainers of freebsdwiki.net currently use and recommend qmail.