How to configure mail for more than one domain?

Hi everybody,

here's my setup: my Lion server sits behind a DSL router on a WLAN and provides OpenLDAP and mail services to a bunch of Macs, iPads and iPhones both for the WLAN and on the Internet (through the router firewall). Internally, I have to use a different DNS domain (e.g. internal.box) due to limitations of the router. The external DNS name for the router (and, through port forwarding, for the server as well) is server.external_one.de. The MX record for the external_one.de domain points to server.external_one.de.

In addition to this, I have two other registered domains, external_two.de and external_three.de. The MX records for those domains also point to server.external_one.de.

I have set up all three domains as local aliases for the mail server. The mail server's domain is external_one.de, and I've configured the mail hostname to server.external_one.de.

The setup works, and mails to all three domains land happily in the user inbox.

Here's the catch: I want my users to be able to choose their sender address when sending or replying to email, e.g.:

• user@external_one.de
• user@external_two.de
• user@external_three.de

How do I have to configure the mail server, OpenLDAP and the Apple Mail program to achieve this?

Thanks in advance!