For the longest time my Sent Mail folder wasn't getting the copies of sent email messages delivered to it like you would expect in every other email client. For some goofy reason, this isn't a default behavior of Airmail. With a little digging through Airmail's preferences, however, I was able to come up with a way to fix this.
- Open Airmail and go to the app's preferences (⌘,).
- On the Accounts tab, visit the Mapping tab on the right-hand side.
- Below the mapping option for Sent emails, check the option for Copy Sent Mail in The Sent Folder.
- Optional: If you use email aliases (like with an iCloud account), you can choose this same behavior by visiting the Alias tab under your account, double-clicking the alias name, and check the option for Copy To Sent.
I hope this helps all of the passionate Airmail users out there who are driven nuts by the fact that Airmail dumps your sent email to the Sent Local folder by default.