I have a primary domain in Google apps and a few domains setup as domain alias. I can recieve emails on username@primary and at username@domain-alias
(All these steps have been completed
http://www.google.com/support/a/bin/answer.py?hl=en&answer=53295)
However I can not find out a way to send emails via the domain alias. (In the compose email window.)
I can add these domains via add account, but then I need to update the smtp settings each time I update the password.