I have a user for whom I'm hosting several different domains. I have created an account for him with the primary domain, and several aliases for his other domains, so that he only has to log into one account to check all of his mail.

However, when replying sometime's he would like the From address to use an alias domain instead of the default primary domain. Is there a way to change the "from" field to something different, so it appears that he's replying from the same address that someone sent mail to?