The first thing that comes to my mind is: how come viva.com is unable to send emails to google workspaces and nobody at viva.com noticed before ? For how long has this going on ?

The second thing is, what email software are they using ? If it was any relatively used software I would not expect this problem to arise (maybe it is some commond software but misconfigured).

Third, while the header is not mandatory, I usually read SHOULD as a "if you don't implement it prepare for possible problems". SHOULD is not MAY.

Fourth, they should be thankful that Google bounced the messages with some appropriate error explaining how to solve it. I have plenty of issues in the past with both Google and Microsoft where they accept the message for then sending to /dev/null