Template:Email/Verify/Doc

verifies that the supplied  appears to be a email address.

This is typically used in other templates to report if they've been passed something that is not an email address.

The optional will be used if everything is alright.

If there is a problem, an error message is displayed and the page is put in Category:Email Error. Currently obvious checks for invalid e-mail addresses (like bad or missing characters) are not implemented.

The optional is used as the name of the template in the error message. So if Template:email calls this the errors can be reported in terms of.