Addendum: this may have nothing to do with your issues, but! the instance feddit.uk had all sorts of trouble with verification emails never even making it to the recipient spam folders, because the emails were getting flagged as spam/suspicious at the provider level, and they were subsequently blacklisted by Spamhaus.
I just looked on Spamhaus and futurology.today is not blacklisted, but you still may be getting flagged as spam at a higher (lower?) level than the user’s inbox filter.
feddit.uk ended up asking affected users to request verification directly via email, which were then manually approved.
https://lemmy.ca/post/3402388