[otrs] FQDN behinded a NATed router?

[Steve Hill]

We have our OTRS server on our internal LAN, which is behind a router 
running NAT.  The actual mail server is outside the LAN, and we POP and 
SMTP mail from there.  The actual FQDN is company.com, and the OTRS 
server is called chuckles, but the DNS is NOT exported beyond the router.

I have tried chuckles.company.com, but of course there is no 
chuckles.company.com on the internet, nor does user at chuckles.company.com 
actually return mail to the correct user.

I have tried chuckles.localdomain, but then I (and other AGENTS) get new 
ticket notification mail with the return address of 
otrs at chuckles.localdomain.

The actual POP3 email address is helpdesk at company.com (which resides on 
our external mail-server) so our normal return mail (notifying the 
CUSTOMER of their assigned, for instance) work properly.

Can someone take all of this, and tell me which FQDN I should put in 
Config.pm.

Thanks,
   Steve