You need to e.g. setup POP/IMAP to receive/send mails (email tickets)
You first need to add a new email address or modify the system email address.
Just go there: Admin Panel -> Emails -> Emails OR scp/emails.php
There you can also specify your SMTP and IMAP/POP settings ;)
Additionally some wiki articles you should check out:
http://osticket.com/wiki/Email_settings
http://osticket.com/wiki/POP3/IMAP_Setting_Guide
http://osticket.com/wiki/Email_Piping