Bug: E-mail validation fails for MX-only domains

The check for whether the domain name in an E-mail address was valid tested whether the domain could be resolved to an A record, which caused mail-relay-only domains, which have one or more MX record but no A records, to fail. I created a new {\tt validMailDomain} function which uses {\tt dig} to query the domain for MX records, filters the result, and returns a Boolean if the domain has one or more valid MX records. The validation of E-mail addresses in both new account creation and modification of an existing account now call this function to verify domain in the E-mail address. (Reported by Joshua Carpoff.)