Disable ⌘Q for a specific application
You can add a Key Command to any app [or all apps] from
System Prefs > Keyboard > Shortcuts > App Shortcuts
I don't have Airmail, but using Safari as an example



Hit the + button, select which app, type the menu name &amp; your chosen key combo
For an app you don't want the original key combo to work for, replace it with something difficult to hit, or something you would never use, like
 Cmd ⌘  /  Opt ⌥   /  Ctrl ⌃  /  Shift ⇧  /  F15   etc

Alternatively, if you want to replace Quit with Close Window, you add  Cmd ⌘   Q   to Close Window instead, [so long as that command appears in the menu somewhere] which will automatically remove  Cmd ⌘   Q  from the Quit item