Postfix email gateway for multiserver ISPConfig

I'm mulling over the architecture for a new multiserver install. To keep it simple, let's assume that we have four servers:
* isp.domain.tld - master ISPConfig server, ISPConfig web interface
* host1.domain.tld - slave server, Apache2, Postfix, Dovecot for customer domains
* host2.domain.tld - slave server, Apache2, Postfix, Dovecot for customer domains
* host3.domain.tld - slave server, Apache2, Postfix, Dovecot for customer domains

In this setup I'd expect a customer on host1 with their own domain example.com to be able to access their site at http://example.com, webmail at http://example.com/webmail, and to set up their email client to send/receive email at host1.domain.tld*.

What would be really cool is if I could setup a Postfix gateway on isp.domain.tld that the customer could connect to instead. Perhaps if it had an alias of mail.domain.tld? Just so much more user-friendly to point all customers to mail.domain.tld than host1, host2, host3 etc. I'm thinking of big ISPs who will give their customers mail.isp.com as their server address, when clearly there are many email servers behind that.

Has anyone successfully done this in a production ISPConfig setup? I like to think that Postfix could be set up to look up the domain for incoming mail in the dbispconfig tables, then relay to the appropriate host server?

And of course I've been concentrating on Postfix here. What about Dovecot?

* Now I realise they could use example.com in their email client, but I've never managed to get my head around how email or FTP could use multiple certificates for virtual hosts. And so connecting to example.com would give a certificate mismatch as the server is hostx.domain.tld

 

For picking up mail that looks like it would be perfect. And then for inbound Postix could do a lookup to route to the corresponding server for that domain. Something I might seriously look into setting up!