I'm in the works of installing a postfix mailserver on a machine currently being used as web server. As of right now, I've got it working as far as that I can send and receive emails using telnet through port 25.
However, as my customer is concerned with downtime, I'd like to setup all the accounts one by one over time, and making sure it works, rather than just cutting the cord to the old mail server and start creating new accounts on the new system.
How can I add the domain customer.com to my mail server and just add something like firstname.lastname@example.org to the users and aliases-lists without the web server trying to send all emails with the @customer.com domain to the new mail server?
I'm running ubuntu, using postfix and postfix-mysql on the new machine, the old mail server is on a hosted environment somewhere else, where I don't have any control.