Check if email address contains one of many domains from a table, ignoring the subdomain
Looking at your current code first, I noticed the following:


You should use email.split('@').last instead of an opaque numeric index (plus, you'll always get a string this way, whereas using [1] will return nil if the address doesn't contain an "@" to begin with)
You should probably downcase the address you're trying to match (provided the university domains are similarly downcased, of course)
You never need to loop, check a condition, and set a variable + break if that condition is true. The reason you don't need to that is because you have Enumerable#detect to do it for you.
This looks like a Rails project, so it'd probably be faster to either let the database check for you, or only load the column you need, instead of loading every single University record.


For instance, to let the DB check it, you can do this

match = University.where("? LIKE CONCAT('%', domain)", self.email).any?
errors.add(:email, "Sorry, no match found") unless match


Since you're not actually loading any records doing this, I'd imagine it's pretty fast (and case-insensitive, so no need for downcasing)

As for matching things in Ruby, Nat's approach is the most straightforward one, I'd say.

I'd still do it a bit differently, though:

address = self.email.downcase

# only load the domain column values, since those are all we need here
match = University.pluck(:domain).detect { |domain| address.end_with?(domain) }

errors.add(:email, "Sorry, no match found") unless match


Of course, you should still check that the address is a valid email address in its entirety.