It’s not about us. It’s about the rest of the world, a large portion of whom uses M365. These blocks mean we can’t communicate with potential employers, family, government institutions, universities, etc.
Here I am, maintaining several block lists (max of 500 entries per list) on our M365 tenant of spam and phishing domains and addresses, and not a one comes from Tuta, Proton, or any other privacy provider. Nearly all are gmail, outlook, and icloud, with a few customs sprinkled in. Their claim that it’s to fight spam is BS.