[CLUE-Tech] SMTP Advice

[Jeff Cann]

OK, I've been running postfix on a server on the net.  I don't have SMTP 
enabled - I've told users to send mail through their own ISP accounts.  
However, they can retrieve email from a secure IMAP daemon on this server.

Now, I think I'm ready to turn on SMTP delivery for the users, i.e., I want to 
relay mail on their behalf but not be an open relay.  I've read about how to 
configure postfix for this situation.  Unfortunately, they are not connecting 
from the same subnet, which looks to be the simplest way to accept a relayed 
mail.

As a newbie admin, I would not like to miss something big that turns my box 
into a spam machine.  Experience is the best weapon for this type of 
situation and I don't have any as a mail admin.  So, I'm turning to our large 
group of clue-tech readers for advice:

1)  What are the best sources of information I should read before I turn on 
SMTP?

2)  What other general advice or 'gotchas' should I know about?

3)  Any suggestions in particular for postfix?

Thanks.