I know this is possible but can't seem to find any documentation on how
exactly to implement it. Pretty much what I want to do is create two
separate GWIAs on a new GW8 system I am setting up, one will handle incoming
email only and the other will handle all outgoing email.

In more detail:
We have an av/spam scanning appliance that sits between the Internet and our
internal GroupWise system. All incoming email passes through this device
before getting to our system and the outgoing email goes directly. The GWIA
on our current system cannot be seen by anything outside our firewall and
that was done to stop spammers from sending email directly to it (and then
to our users) avoiding our av/spam solution. We are now trying to open up
access to our email system from other clients/devices (3rdparty email
clients, smartphones, etc) over IMAP. However we'd rather not open up port
25 on the GWIA (for sending email) if it allows both incoming and outgoing
email for the above stated reason. Instead it seems smart to just run two
GWIA's, with "GWIA-Incoming" only accessible from our av/spam appliance and
"GWIA-Outgoing" seeable by the world but only able to send email externally
and not accept anything.

I am pretty sure this is possible, but am just looking for some
documentation to get me started in the right direction.