Wow, that's really worrying that an autoconfig file could even do that!

We have had reports of the autoconfig being annoying if you need to change any settings because it locks some of them in place, but I've never seen any mention of that kind of crash.

Regarding the re-authentication messages, I suspect the reason is that you have migrated calendar events across into Fastmail from those other systems, and those events contain links to authenticated resources which your Mac is trying to load. This isn't really something we can fix other than stripping those out of your events, but that has its own downsides!

I didn’t migrate anything into Fastmail.

When I removed the Fastmail accounts everything else went back to normal. It could very well be a problem with MacOS.

When I have time I’ll try setting it up again and see if it happens again.

Works fine on my iPhone.

If you do try it again, we'd be keen for you to contact our support if you have problems so we can look into if there's anything we can do at our end.

Were you just using Fastmail as a copy if Gmail? Or were you manipulating gmail through jmap?

Gmail and Fastmail were not linked in any way. I have had my Gmail account set up in MacOS for years. I added my Fastmail account. My system became unstable and all my Internet accounts started prompting for logins.

I've been using Fastmail, on my Mac, for six-plus years now, no problem.

Didn't use any sort of auto-configure script, though.

That sounds unreasonable. Did you directly download the profiles (.mobileconfig) from Fastmail?

Yes, directly from fastmail.

Not sure if MacOS tries to use JMAP or if this was IMAP. The system was completely unusable most of the time. I was able to delete the accounts between lockups. Didn’t do any troubleshooting beyond that.

Don't think MacOS has JMAP support at all, so most likely IMAP. Happy Fastmail user here. Used it on a MacBook up until about 2 years ago with no issues with native Mall.app. Used it on my iPhone until I switched to Android about 3 months ago—also no issues with the iOS configuration script which sets everything up on iOS with pretty much one click—very useful also for setting up family members' phones. Found Fastmail much, much better than Gmail IMHO who has the worst IMAP implementation I have ever seen. Bugs include command like deleting/moving/etc mail not actually having any effect over IMAP. Gmail only works in macOS/Outlook/etc apps I suspect because of all the workarounds Apple, Microsoft et al have implemented to paper over Gmail's IMAP bugs. Fastmail by contrast has worked as expected on all platforms, although I now just use the Fastmail app on Android and the Fastmail web interface on Firefox/{Mac,Windows} as Android/Windows's native apps are horrible compared to their Mac counterparts.

Yeah Fastmail works great on my iPhone, both their app and the native one. Setup was a breeze.

MacOS setup wasn’t hard to do, it just made my system unusable.