Verification and spam
I set up a website recently - for COVID-related reasons, to manage reduced capacity in a series of venues by taking advance booking.
Sign-up results in a verification email being sent. A significant number of those emails go into the spam trap. Not because they are phrased in any spammy way - online checkers confirm that they are not - but apparently because they don't like new email addresses with confirmation links in them.
It's all very well asking people to check their spam folders, but many less experienced Internet users (and COIVD has made a lot of people use the Net in new ways) don't actually know what that means, or how to do it. Email providers hide what they judge to be spam away, for good reasons.
So I have to send a reminder/offer of help out to a sizeable batch of people weekly, and manually confirm a lot of accounts when they respond. It's a pain.
Perhaps the email giants could come up with a better system!