The funny thing is that there’s absolutely no mention of it in the help files, the UI, or any where else I can see. Here’s Apple’s help on the ‘Email Address’ field:
Email Address: Enter your full email address. This will be the From address in your outgoing messages. For example, firstname.lastname@example.org.
What they don’t tell you is that you can associate multiple email address’ with each mailbox/account by separating them with a comma. This is great news for someone who has multiple email address’ all forwarded to one mail box (joe@, joe.smith@, admin@, etc). Here’s a screen grab:
As a side note, Mail.app is really smart about which reply-to address or account it should use for new messages and replies. For replies, it will attempt to reply with the same email address that the original message was addressed to (if you have it specified as an account or alias). For new messages, it will use the main address for whatever account you currently have selected in the “mailboxes” drawer. Wonderful!
Update Oct 11 2004: Ben Summers points out that if you hover over the Email Address field, the tool tip does in fact mention this feature. It’s still way too obscure for my liking, but at least it’s a documented feature!