Postfix config

[Tenant <tenant@tenant.net>]

We have a server which is still running Sarge (we'll go to Lenny when 
it's released), but I don't think that's relevant here.  In the 
current server, we're running Sarge, Apache/1.3.33 with a fair number 
of virtual hosts and Postfix.

I'm trying to set up a number of email accounts for the various 
domains we host. We used to run RedHat and Sendmail, so we're 
familiar with that (virtusertable, genericstable and aliases), but 
not with Postfix which seems not to use all of those files. We also 
using Webmin, although when necessary I use the commandline.

I've set up user accounts and somehow it knows, when I can mail from 
an outside source, it comes in as myaccount@domain3.com.

But when I mail from user3@domain3.com back to the outside email 
account, it comes in as user3@domain1.com (the machine's main domain) 
and not from domain3.com.

I checked aliases, but that does not seem to be the right place. We 
already have emails set up for domain2.com which works fine. The 
mapping for that is not listed in aliases. So I'm at a loss as to 
where to map user3 to domain3.com for both incoming and outgoing email.

I've looked at all the text files under /etc/postfix and can't seem 
to find where this would be.

I hope this made sense. Thanks for any help you can offer.

TT