550 5.1.1 and 550 5.1.10: Exchange Online cannot find the recipient
550 5.1.1 and 550 5.1.10 mean the recipient address does not resolve in Exchange Online. Check for deleted, unlicensed or mistyped mailboxes first.
550 5.1.1 and 550 5.1.10: Exchange cannot find the recipient mailbox
550 5.1.1 means Exchange resolved the address but found no mailbox. Learn why recipients vanish after moves or sync gaps, and how to fix it fast.