The Id of the message.
The Id format is as in RFC 2822: id-left@id-right .
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.
You may insert the keyword "%MessageId%" anywhere in the value to have the adapter generate and insert a globally unique identifier.
This property is not available in the Receiver.