Thomas Lee is correct in that a quoted local part is valid in an email address and certain mail addresses may be invalid if not in a quoted string. However, the characters that others of you have mentioned such as the umlaut and the agave are not in the ASCII character set, they are extended ASCII. In RFC 2822 (and subsequent RFC's 5322 and 3696) the dtext specification (allowed in quoted local parts) only allows most ASCII values (RFC 2822, section 3.4.1) which includes values in ranges from 33-90 and 94-126. RFC 5335 has been proposed that would allow non-ascii characters in the addr-spec, however it is still labeled as experimental and as such is not supported in MailAddress.