Today we were surprised to find out that you can use apostrophe symbol in email address. That means that email address like o’ is absolutely valid. Btw, note that even wordpress does not recognise it as valid.

Which brings me to the RFC and Wikipedia article about email format. Ready to be surprised? Here is list of absolutely valid email addresses, according to standard:

Keep that in mind when you are implementing email validation function in your software! Or better way, use something already implemented but check it against these extremes. Or at least check it on email with apostrophe in local part.