[Nursie]

Usually, for the statutory thing, you do have to be able to prove the counterparty received the communication, so registered mail is often the best way, because someone has actually signed for it at that point.

But if they respond at all using other channels, you probably still have enough.

[alkonaut]

Seems GDPR is pretty explicit since it requires specific documentation (Data Protection Statements/Privacy policy) and explains that they should contain complaint contact information. I think it would be pretty easy to contact Microsoft on the "expected" channel for a GDPR complaint.