First off, thank you for your work on Geary. I just started using it yesterday and saw that there wasn't easy account setup for FastMail, so I added it.
I added FastMail as the first entry on the list based both on alphabetical order (which I believe the list is in). It also makes sense to list providers by ethical design criteria (https://ind.ie/ethical-design) so that those with surveillance-based business models (like Google, Yahoo, etc.) are not being promoted.
I'm still very new to Geary, Vala, and to the codebase and I'm not sure if I've added the “special folder” mappings correctly (https://gitlab.gnome.org/aral/geary/blob/add-fastmail-service-provider/src/engine/imap-engine/fastmail/imap-engine-fastmail-account.vala#L35) as I got warnings and certain folders did not appear when I first ran Geary and set up my account but all folders loaded in the second time and the account does seem to be working properly.
While it was pretty straightforward to add a new provider, it would probably make sense to look into autoconfiguration going forward (e.g., along the lines of https://developer.mozilla.org/en-US/docs/Mozilla/Thunderbird/Autoconfiguration). I'll open an issue about that if there isn't already one when I get a moment.
Looking forward to hearing your thoughts and hopefully getting this integrated at some point. Please let me know if there's anything I can do from my end to help speed that up or if any changes are necessary.
Thanks again for your excellent work on Geary.