To cut a long story short - how do you define multiple “From:” addresses for an email account in Mail.app?
I always thought you'd just have to create a separate account. But what if the 2 accounts are actually on the same server? Mail.app won't allow you to create two account with the same incoming server host name.
It turns out this is all very simple - just add the From: addresses to the Email Address: field in the Account Information tab, separated by commas.
That adds each one of those to the From dropdown menu of the New Message window. I came accross this because Apple did it when MobileMe came out.
This might not sound like much but it's a really cool solution to a potentially icky problem. Like having a mail server receive mail for multiple domains and suddenly finding out your users want to send mail from multiple domains…