[golem14]

Hilarious - German users lecturing Google on how to interpret the English RFC?

I say this lovingly, having significant German ancestry:)

But taking a step back :

did viva previously send message ids and pushed a change to prod to strip it? Was it on purpose or an accident?

And other email providers like proton or Hotmail - do they accept messages without message ids?

Have other clients of Google workspace complained about this issue?