Postfix, dovecot mail gateway
I'm trying to setup a mail gateway using Postfix and Dovecot but so far I've only gotten one-half of the process to work (can successfully send mail out, but can't recieve). The setup I want is to have a mail gateway that can forward mail to back-end imap mail servers--this way I can scale the webmail as I need to make room for more users. I'm using Squirrelmail as the front-end and have configured it to look up the appropriate imap server for each user as they login but, I'm not clear on the setup to allow for the forwarding of mail to the appropriate mail server for each user. These users will all be within the same domain--just there imap server will be different. Not sure whether I need to setup a transport map or virtual map or both. Can anyone help? I've tried the setup as described in the 'mail_filter_gateway' howto but must have missed something somewhere.
My DNS MX record points to the mail gateway host and I've configured my routers NAT to allow the appropriate traffic through to the gateway.