I am in love with the Fantastical and Things 3 integration. Besides, my company's exchange account does not offer draft folder, I lost countless mails because of that. Airmail comes to help in that respect as it offers a draft folder on its own (I guess it uses iCloud for that).
Many of AirMail 3 features can be found in a free app Spark but the crucial ones I care are only special to Airmail:
1. Hand-off between mac and iOS (Spark offers it only between iOS),
2. Integration with Fantastical and Things 3, it is super useful to select a text and send it to these apps, gotta try it to believe it (Spark will never support this, because the developer (Readdle) is in
competition with Flexibits in the smart calendar app market, on a separate note Fantastical beats the crap out of Calendar 5 btw),
3. The “zoom" feature in a mail in iOS, (Spark does not support this for some ridiculous reason),
4. Proper support for .mime attachments (Spark sees them as attachments, even though they are just text in a mail form, you need external to view them, even stock apple mail sees them as text,
this frustrated me a lot with Spark because I get weekly emails containing .mime attacments),
5. Marking messages as "To do" or “memo” other than just “Starred”. (Last time I used it spark did not have this feature, maybe I missed it)
6. VIP notification support (no clue why spark does not allow this, their e-mail classification algorithm is not very well trained, iphone classifies one thing mac classifies another, but they keep their
notification system based on that classification. People send me personal e-mails, dont mean that they are as important as the ones that I am getting from my boss during daytime, note though
that apple mail supports this already)
7. Non-VIP banners, I kinda like that when there is new e-mail from a non-VIP notification pops-up as a banner on the top right corner of my mac, without any notification bell ringing. That informs
me that there is mail to read but also tells me that it is not someone that is important.
8. Muting or Blocking a mailer, this is increadibly useful! (Spark does not have this afaik)
9. I also like the unseen mail counter on the menu bar. It turns blue and it is really sweet. (Spark does not have this, nor does apple mail)