As a personal problem example (what else from me, right? ;-) I have some email addresses which work fine but are sometimes rejected on registration forms.

One is a personal email address of the form '_\w+_\@populardomain.example', and gets rejected for the underscores or the leading underscore. My company also holds four domains of the form '\d{2}[a-z]+\.[a-z]{3,4}'.

I get somewhat frustrated with sites that refuse to accept those. It's quite alright in the long run if they offer an alternate way to get registered, such as emailing a support contact or leaving a note for registration support through a form when the address is rejected. Both of those take human intervention, though. I can always use a different email address to sign up for something, but I generally use certain ones to group certain kinds of topics. If I don't find an address that works through some method in a couple of tries, I usually start looking for the competition's website.