Buying a seat for Exst consistently results in this post-flight email from AA:

Last chance Don't leave your miles behind Our records indicate that you are enrolled in the AAdvantageŽ program.

You can request miles for the trip you just completed in one simple step.

My wife and my miles are being posted just fine, so I guess they're referring to the Exst, and the trigger for the bit in bold is having my email address in Exst's record. Probably I could create an account and get miles credited to it with never a problem, but AA says that's against their rules so I won't.