Does Fastmail.FM have the equivalent of the "Archive" action in Gmail? Mail.app on OS X and on iPhone seem to play well with Archive and it's not something that I really want to give up.
Here's a screenshot from Mail.app: https://files.app.net/1/79773/aTOWGMJ-rQ2Js_Edg-CWLi5FsLEVYq...
I either never knew that existed when I tried Mail.app with Gmail or it didn't work. In any case, it works perfectly with FastMail's "Archive" folder.
In our web interface, there are two buttons - one moves to Trash, the other to the Archive folder.