It might be reasonable, but highly annoying if you run a, say, sign-up double opt-in or an email-based second factor/OTP. Customers don't like waiting an hour or whatever greylisting period, and so as a SaaS operator you are all but forced to go to one of the big e-mail senders like AWS SES because you don't stand a chance otherwise.
Ah, darn... I'm starting a web app with email verification now, using my own email server for sending and I didn't consider this. Thanks for the warning. Guess I'm about to find out how bad it is.
Pretty annoying to test for this reliably as well...
