email: Message-Id header with user and thread ID

This patch suggests a change to the message-id creation process that includes stamping the receiving user-id (staff or client) along with the thread-id of the originating notice. This allows detection of threading if the clinically brain-dead mail client drops all the other header detection mechanisms, including the tag placed in the email body, on response.

This patch works for both client and agent communication.

*Hopefully this will make the next level of threading detection.*
`$rv['code']` should be `$parts[0]`?