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].
Domain:
The Internet standards (Request for Comments) for protocols mandate that component hostname
labels may contain only the ASCII letters a through z (in a case-insensitive manner), the
digits 0 through 9, and the hyphen (-). The original specification of hostnames in RFC 952,
mandated that labels could not start with a digit or with a hyphen, and must not end with a
hyphen. However, a subsequent specification (RFC 1123) permitted hostname labels to start with
digits. No other symbols, punctuation characters, or blank spaces are permitted.
