The Id of the message.
The Id format is as in RFC 2822: id-left@id-right .
A unique Id will automatically be generated on startup. Sending a message will reset id-left if the MessageId has been used in the previous message.
If you set MessageId to a string of the form "@(id-right)" a unique id-left will be generated. If you set MessageId to an empty string, a new MessageId will be generated with the same id-right.
After an MDNReceipt is returned or set, MessageId will contain the Original-Message-ID found in the MDN Receipt.
This property is not available at design time.