This class allows the developer to validate e-mail addresses according to RFC 5321, RFC 5322, or custom requirements. It can also check to see if the domain can be resolved to MX RRs. If validating according to the more expansive RFC 5322, e-mail addresses may contain one of a dot-atom, a quoted string, or an obsolete local-part, either a domain name or a domain literal (IPv4, IPv6, or IPv4-mapped IPv6 address) domain, and nested CFWS (comments and folding white spaces).