/
(?(DEFINE)
(?<plain>[a-zA-Z0-9#!$%&'*+-\/=?^_`{}|~]+)
(?<quote>"(?:\\"|[^"])*")
(?<forbidden>(?<![.+]))
(?<local_part>(?"e)(?&forbidden)|^[^\.](?&plain))
(?<domain>[a-zA-Z0-9-.]+\.[a-zA-Z]{2,63})
(?<mail>(?&local_part)@(?&domain))
)
^(?&mail)+$#.. will not handled
/
mx