Then it's the spam detectors not liking the format of the email sent by Flarum.
Can you run a test email with this service please:
https://www.mail-tester.com/
Let us know how many points you receive. I'd like to hone in on if the formatting is a minor or major issue here. If your score is 9-10 and getting sent to spam then that's an issue. If it's iffy then simply improving your score may resolve this.