Google Apps handles our mail, and relays any non-existant accounts to our Postfix server. This works fine, and we can then use virtual accounts on our Postfix server to do whatever we wish.
However, we have a problem with relaying mail from inside our LAN through our Postfix server to addresses at our domain - it determines it's the local server, tries to check if the user exists (it doesn't, it's only in Google Apps), and then bounces the mail. Currently we are working around this by adding every single GApps account to the virtual mailboxes and putting in an override in /etc/postfix/transport to force it to send mail to GApps for those accounts, which is a pain. Also, email addresses like someone+someth...@example.com have to be entered specifically, Postfix won't just send all someone+*@example.com to some...@example.com. It seems like we should be able to say instead "use MX record for entire domain" as a default, and then override just the accounts that should be on the Postfix server to be locally delivered. Alternatively, have some way in which if it doesn't exist locally it tries to deliver via MX, making even simpler configuration?
