RFC 5322 Support for is_email() function. Quick fix.
I was quite suprised when I tested his email something+something@anything.com and found out that the default WordPress function is_email() returns false for this type of emails. In other words, is_email() function isn’t able to validate email addresses with a “+” in it but those are valid email addresses as well as per RFC standard 5322. By the way, if you look at the official documentation of is_email() function, there also said that it is not RFC compliant.