First of all an appeal on all specialists to be understandfull with me, newbie. I have a RaQ 550 that I have to admin bymyself. I have executed mailq and found out that there are mails in the queu that I did not sent. Is someone exploiting my server? Here is an example of the report:

----Q-ID---- --Size-- -----Q-Time----- ------------Sender/Recipient------------
mBRIDxxxxxxx 13270 Sat Dec 27 10:13 Majordomo-Owner@xxx.xxx.net.
(Deferred: Connection timed out with attcanada.net.)
xxx@attcanada.net
mBR7Hvxxxxxxx 12986 Fri Dec 26 23:17 Majordomo-Owner@xxx.xxx.net.
(Deferred: Connection timed out with ifg.com.)
xxx@ifg.com
etc.
I do not have smtp active, only pop. I have activated now POP Authenticated Relaying with my IP through the GUI interface. Would this be enough to prevent exploiting my server for spam?

The first thing to try would be something like: Email Server Test - Online SMTP diagnostics tool to see if you are suffering from any of the normal exploits.

Secondly make sure you don't have any web based services ( browser forms etc. ) that could be used to submit mail, these can easily bypass authentication by sending as a local user if not properly secured.