Has had a terrible bug for a while with no fix
This app has potential but until they fix some of the bugs I recommend that you stay away from the current version. In particular if you delete several messages in fairly rapid succession - which is pretty normal if you get lots of promotional e-mail - then you can easily get into an off-by-one state where the message shown in the inbox list is not the same as shown in the full message view, but is the one before or after. This makes it very easy to inadvertantly delete the wrong mail message.
I reported this bug seven months ago (ticket 7116) and the developers acknowledged that they had several other reports o the same issue but it is still not fixed.
Gramsterix about
Airmail - Lightning Fast Email, v2.6.1