I was just notified of 25 unsubscriptions from the Wikidata mailing list (and 2 on WikiIT-l). They were probably all "real" bounces, and mostly they were former WMF or WMDE staffers whose mailbox was presumably disabled, but I saw some addresses that I know for sure are working.
Google, Microsoft and others are known to lie about mail delivery. It's possible that by sending an unusual amount (or rate) of email to some non-existing addresses we got somehow treated like spammers, and some messages to existing mailboxes were treated as the kind of spam that is bad enough to bounce back with a lie but not bad enough to be silently discarded. I have no idea how to debug this for Google or Microsoft mailservers but I've suggested one user whose email is on a personal domain name to file a bug with any logs they might have. I hope that's fine.
Federico