I’ve asked Airmail help center and they told me that Airmail wasn’t well supported by Drafts 4. Same goes for Drafts 5.
I found an action which kind of work, but the font I usually use and set up Airmail with is never used when the message has been written on Drafts and then sent to Airmail… I have to select the whole text and set another time the font I want…
It’s kind of a shame!!!