class Mail::Message

def message_id=( val )

mail.message_id #=> '1234@message.id'
mail.message_id = '<1234@message.id>'

the < > usually seen in the mail header, so this method will return only what is inside.
Sets the Message-ID. Note, per RFC 2822 the Message ID consists of what is INSIDE
def message_id=( val )
  header[:message_id] = val
end