Now let's start to make the server do something useful (apart from acting as a room heater).
The mail services for Ubuntu are based around postfix for SMTP and dovecot for POP3/IMAP.
Fetchmail will be configured to collect email from external sources such as Gmail and your ISP via POP3.
In order to minimise the risk of spam/virii being received, it is well worth considering the installation of suitable measures. These are provided by spamassassin, clamav, amavis-new and postgrey.
The optional mailgraph package will provide an overview of the performance of the mail system.
If required, various webmail packages are also available, such as squirrelmail and roundcube