class Mail::Message

def message_id( val = nil )

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

Also allows you to set the Message-ID by passing a string as a parameter

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

Example:

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