Answer
GMS and a remote server or client had identified that they are attempting to resend a message which has already been partially transferred. GMS is telling the remote system how far the previous attempt to send this message got so that both GMS and the remote system can continue where they left off. This saves considerable bandwidth and is especially effective over slow or over-loaded Internet connections.
In this case, session Undefined is processing a connection from Undefined which was processing email apparently from Undefined for Undefined (if there are multiple receipts for the message, this is the first recipient). Last time Undefined bytes were successfully delivered so the remote server will continue to deliver the remainder of the message.
No action is required by you except to congratulate yourself on saving bandwidth!
See Also:
Keywords:CHECKPOINT transfer recovery bandwith RFC1845