I send a low volume of "critical" emails from my FastMail account, and have been for a number of years. I never received any reports of them ending up in spam.

I send a low volume of "critical" emails from my FastMail account, and have been for a number of years. I never received any reports of them ending up in spam. It's unfortunate that many email providers no longer provide a detailed reason why something was marked as spam, but considering you say it's with several providers I'm inclined to suspect it's something non-obvious in your setup or email content somehow.