Because Mail cannot do that? That's right... Mail does not know how to filter sent mail on its own. With a little help from the Mail Act-On add-on, Mac OS X Mail can save all your outgoing mail to your universal "Archive" folder, for example, file to correspondent or project mailboxes, delete certain messages, set colors, or even run AppleScript actions — all according to your rules and criteria.
Filter Outgoing Mail in Mac OS X Mail (with Mail Act-On)
To have Mac OS X Mail filter messages you send automatically:
- Make sure Mail Act-On is installed.
- Select Mail | Preferences... from the menu.
- Go to the Rules category.
- Now open the Outbox Rules tab.
- Click Add Rule.
- If you have already set up an incoming rule with the same or similar criteria (or actions), note that you can copy it: go to Inbox Rules, highlight the desired rule and click To Outbox. Make sure you edit the rule, though — likely, you'll have to exchange "Any Recipient" for "From", for example.
- Select the desired criteria for spotting the right messages for filtering under If ___ of the following conditions are met:.
- Make the criterion read "If any of the following conditions are met: Any Recipient Contains maya@example.com", for example, to filter all messages you send to (but not necessarily only to) maya@example.com.
- Pick the desired actions to be applied automatically under Perform the following actions:.
- Make the actions read "Move Message to mailbox: Archive", for example, to file the sent message automatically not in the Sent folder but in "Archive".
- Click OK.

