For a while now, I've been wondering what the Open Directory feature in Airmail does. I've always left it authenticated to my OS X Server, but never really understood how this enhanced the application.
I finally had a moment to contact the developers tonight and they advised me that the Open Directory feature is currently enabling auto-complete of contacts in given user's address book, which is stored in the Open Directory record on OS X Server.
I'm very glad to see that the developers of Airmail gave Server integration enough thought to include this in the app. I did ask if they would be including the ability to use a public dropbox folder on an Open Directory server and they confirmed that the enhancement is in the works:
What are you asking is a sort of using a custom ftp as document uploader, and then create a public url... that is planned Thanks. [sic]
While it may not specifically integrate directly with Open Directory authentication methods on OS X Server, it is an enhancement I will be looking forward to. I'd rather run my own cloud versus trusting my data to third-party cloud services like Dropbox or Droplr.