[exim] OT: General question about dmz and email gateways

Just wondering what is considered best practice/thoughts of email setups.

1) SMTP gateway sits in the DMZ for the company and forwards mail
through a pinhole to the email server in the inside network and vice
versa. The gatway might deal with spam and av issues.

2) Machine on the inside does everything, external servers connect
directly. Internal machine does AV/SPAM, pop3 for internal users et
al...

btw this is a 450 person company.

I would have thought option two would be a 9 on the risk/load scale?

Thoughts...?

Cheers
Kristian