That’s a shame.
I am going to send them a personal email pushing the compatibility.
If you are making an major email software for Mac, it should be compatible with the most popular spam software.
Airmail shows the most promise out of any recent email attempts (outside of Sparrow), but I am rather disappointed by how slowly the software has progressed and how long it takes for the developers to implement the most essential features.
I have been using that software since it was in early beta for many months now, and progress is painfully slow.
Let me see if I can push things on my end. I am only one user out of many, but perhaps I can become a vocal minority.