What does phx.gbl mean (in the Message ID)?

"phx.gbl is an internal Active Directory domain that Microsoft uses to manage datacenter machines.  It gets added to message headers as a way to trace what machines actually saw a message when Microsoft needs to diagnose specific problems.  The name is equivalent to a *.local domain some organizations use for AD management.  It is by design that these machines are not addressable from the internet as part of Microsoft's defense in depth security planning."

Jomi Schell, Principle Developer Lead

Updated 24 Dec 2011 · Upvoted by Martin Handwerker, Development Manager at Microsoft for a decade and Anjor Hemant, Software Engineer at Microsoft

https://www.quora.com/Network-Protocols-Where-does-phx-gbl-in-all-Microsoft-e-mail-headers-derive-from