Two complaints are: lack of Markdown support (which the desktop client has) and not handling image attachments properly (some emails are received without any indication of an attachment). Attaching an image to a plain text email in Airmail or receiving a plaintext email with an image from someone else does not always trigger a paperclip attachment icon. Not handling MIME content correctly via RFC standards is why this app gets 1 star which developers claim is by design. Developers refuse to fix bug.
teaneedz about Airmail - Your Mail With You