Allow email domains be black/whitelisted? Perhaps to a single domain?

Ok, good idea, work in progress. If someone tries to sign up with an email address from a not-allowed domain, then, the simplest thing is to directly reply "Forbidden" and not let the person proceed with creating an account at all. What do you think about that?

You'll be able to type a list of allowed domains. Also, if in some rare cases, someone needs to join, but s/he has an email address that breaks the someone@some.domain.org pattern — then you can add that exact email address too, to the list of "domains".

(For others who might wonder what's the use case for this: This is useful for organizations where all members have an email address on a certain domain, like student-name@some.university.org, and the community is for those people only.)