In some countries, there are allowed to use extra characters in the user part of email. User can define local characters.

[BartnickiBartlomiej]

In some countries it is allowed to create emails like Łókaść@gmail.com.
My proposition is to let user define allowed local characters.

If you let me permision to create branch I can create pull request.

https://pastebin.com/09n9J2UB