How to disable expiration?

Hello everyone,
I'm newbie wirh inn. I've installed version 2.3.3 on RedHat Linux 8.0 and I'm 
setting a private news server with two newsgroups.
At the moment, I'm setting functionality: I see that messages expires after a 
certain time; there is a way ti disable completely expirations?
This is necessaty because this news server is a sort of "mail-to-news gateway" 
and should take history of emails received.
Thanks in advance

Alessandro Avidano