At my company, we recently migrated from 2013 to 2016, but had to keep a 2013 footprint around for support of older Blackberry 5 devices. In our environment, we have a SendMail farm that is an internal open relay as well as being responsible for the send and receiving of external mail. We used various config files on this SendMail farm to distinguish if an email came from Exchange and not one of the various internal application servers that relays off this farm. One of those methods was using the MessageID header of the email. In 2013 that message header would look something like GUID@ServerFQDN, as explained in this technet article. The same article also states this is true for 2016, but this does not appear to the case in our mixed environment. While 2013 still follows this pattern, 2016 instead uses the primary SMTP address of the sending object for its domain on the MessageID header, so GUID@Primary_SMTP_Address_of_Sending_Object. Whenever the primary SMTP address of an Exchange object changes, and that change is reflected in the Global Address Book, any message sent by that object will be in that format. I’m guessing that this was a change meant to help better track messages in Office 365, but I’m curious as to why this behavior change hasn’t been called out in any of the Cumulative updates for 2016 or the official documentation.