At some point I switched from using the mail client ("Mail") that ships with ElementaryOS over to Geary. I don't remember exactly why, and now I've managed to screw-up my install of Geary so this morning I re-installed Mail and gave it another shot.
It may have had to do with a weird issue where the HTML that Mail generates includes an "editable" indicator that makes the posts I write using it appear editable on the web (even though there's no way to actually edit them). So I'm writing this post mostly just to test that.
I hope that's not the case because if it is, Mail isn't going to be very useful to me until that's resolved.