what characters are allowed in an email address
uppercase and lowercase Latin letters A to Z and a to z;
digits 0 to 9;
special characters !#$%&'*+-/=?^_`{|}~;
dot ., provided that it is not the first or last character unless quoted,
and provided also that it does not appear consecutively unless quoted
(e.g. [email protected] is not allowed but "John..Doe"@example.com is allowed);
space and "(),:;<>@[\] characters are allowed with restrictions (they are only allowed
inside a quoted string, as described in the paragraph below, and in addition, a backslash
or double-quote must be preceded by a backslash);
comments are allowed with parentheses at either end of the local-part; e.g.
john.smith(comment)@example.com and (comment)[email protected] are both equivalent
to [email protected].