Yay! I am so happy to hear this. Congratulations and thank you to the Fastmail team and people at Apple who made this happen. This has been my only gripe as a Fastmail customer and the only reason I haven't been using Fastmail as my main email account yet. Very cool :)
I still agree with these notes: https://news.ycombinator.com/item?id=9664594
I also think -- and I've tried almost all of them except outlook -- that it's the best non-gmail webmail available today. But that's a big caveat since gmail is free except for your privacy.