I can't say for certain, but this looks very much like a form of bounce-validation being implemented by the websence.com domain mail gateway. The ironport does something similar if you enable the bounce validation.
Note that the envelope sender does not have to be the same as the message From header, which probably still shows the email@example.com address. The envelope sender is used more for NDRs and similar.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...