2FA has never been an issue with me. If it's a one-time thing (like signing up for a website) the wait doesn't bother me, and no services have ever rejected me

2FA has never been an issue with me. If it's a one-time thing (like signing up for a website) the wait doesn't bother me, and no services have ever rejected me because I didn't respond within five minutes. If it's a recurring thing, I can always whitelist the domain, the IP, the sender, or all three. Even the senders with pools of mail servers hasn't been an issue---the email will eventually get through. As far as I can tell, in the fifteen years I've been using greylisting, that has just not been an issue. When I first started with greylisting, I set the embargo time (minimum time to reject emails) to an hour. I later dropped it down to 25 minutes, but that's just me being a bit over-cautious. Just a one-time temporary reject is enough to filter out something like 80% of what I catch with 25 minutes.