for a really good article on apple mail’s most underused features by joe kissell, head here on macworld.
the ones we found most useful are:
3. Multiple From addresses
If an e-mail account has more than one valid From address (for example, if you use email@example.com as an alias to firstname.lastname@example.org), you can configure Mail to let you use any of those addresses when sending a given message from that account. First, choose Mail -> Preferences, click on the Accounts icon on the toolbar, select your account, and click on Account Information. Then, in the E-mail Address field, enter all the addresses you might use with that account, separated by commas. Close the Preferences window and click on Save. Now, when you compose a new message, you can choose any of those addresses from the From pop-up menu below the Subject field.
6. Send Again
Suppose you sent a message to a bunch of people and you realize you forgot someone. Instead of opening the message in your Sent mailbox, copying its contents, and pasting it into a new message, just select the message, choose Message -> Send Again (Command-Shift-D), and replace the existing recipients with the new one(s). The subject and message contents remain the same as before (although you can edit them if you like).