I figured out a way to use Mail.app with a very specific use case:
- You want to send mail from multiple addresses, e.g. you forward email from a secondary account to your primary account, but still want to send mail from both these addresses.
- You want to use the correct SMTP server for outgoing mail, but you want your primary account's Sent folder to have a copy of all your outgoing messages.
Mail.app on Mountain Lion (OS X) gets most of the way there by default. You can set up two different accounts, and it will intelligently select the proper outgoing address for replies.
When it does this, it will use the SMTP server you specify for each account. The problem is that when you send mail from the secondary account, it won't end up in the Sent folder for your primary account. Instead, it will be in the Sent folder for the secondary account. This is bad if you're trying to keep everything in one place.
The solution I found is actually surprisingly simple. The trick is to bcc yourself on all outgoing messages:
Then, set up two rules:
- Incoming messages from your primary address → Trash
- Incoming messages from your secondary address → Sent folder on primary account
Here's an example of the 2nd rule ("Fastmail" is my primary account; the email address for my secondary account is blurred out):
When I send mail with my primary account, the messages is automatically put in my Sent folder, so I just discard the bcc'd message. That's the first rule.
When I send mail with my secondary account, because I'm forwarding mail from my secondary account to my primary account, the bcc'd message is forwarded to the inbox of my primary account. The second rule above moves it to my Sent folder.