Mail Services Newbie - Selective Relaying?

Question

Hello everyone. I'm fairly new to OS X Mail Services, so please forgive my naiveté when asking this question. I'm a CS professor at a small college, and we recently purchased and installed an Xserve for use by our department's students. Everything is working fine with the exception of mail services. The problem is that, for local emails (@ourschool.edu), I need to relay through our (separate) SMTP server. This works fine when sending mail to a local address. However, when sending email to an outside address (another other than @ourschool.edu), I do not need to relay the messages through our SMTP server.
Is there a way to configure Mail services to "selectively" relay messages based on their destination domain? Basically, if the destination is @ourschool.edu, I want to relay it. Otherwise, just send it to the destination directly.
I know that one solution would be to open up our SMTP server for total/complete relaying from our Xserve, but I was wondering if I could accomplish the same thing without the need for relaying everything.
Thanks!

Answer 1

You cannot do this via the GUI, but if you do not mind using Terminal, this can be achieved by using transport maps.
There are several ways of doing this, but roughly it goes like this:
Make a copy of /etc/postfix/main.cf first.
Edit /etc/postfix/main.cf
comment out your relayhost setting if you have already set it:
#relayhost = mysmtpserver.mydomain.com
add:
transport_maps = hash:/etc/postfix/transport_maps
create a file:
/etc/postfix/transport_maps
with these contents:
ourschool.edu smtp: mysmtpserver.mydomain.com
otherdomain.edu smtp: othersmtpserver.mydomain.com
save it and issue:
sudo postmap /etc/postfix/transport_maps
sudo postfix reload
For more info, look at the transport maps examples here: http://www.postfix.org/STANDARDCONFIGURATIONREADME.html
HTH,
Alex
P.S. The forum software mangles above sample a bit, so make sure you check the link. mysmtpserver.mydomain.com should be in square brackets and there is a * before the last colon