I can register on this forum using my regular gmail account, but I'm only using that for Google's junk, so I won't see any notifications easily. I want to change my address to my actual e-mail address, but it won't let me. It says my e-mail address is invalid, while it's perfectly valid. It even receives e-mail (from every other service/provider) perfectly fine.

This is not it, but produces the same error:
john+zenphoto@example.com

Why is this invalid? Answer: it's not.

Please, zenphoto devs, correct this behaviour for those with a cool e-mail provider.

We are sorry about that. We will try to take a look but for all the forum software is old and scheduled to be replaced in the hopefully not to far future.