We have been operating our Office 365 tenancy successfully for several years. However, one user has recently started to experience problems receiving mail from addresses outside our domain. Any message sent to this user is rejected, with the sender receiving an NDR:
The error that the other server returned was:
550 5.4.1 [recipient address]: Recipient address rejected: Access denied
This affects only this one user and not anyone else in the same domain. There is nothing unusual about this particular user's account - she has the same setup as everyone else, her mailbox isn't full, it isn't delegated to any other users, has no forwarding, no message delivery restrictions, is not a hybrid account (we've always been purely Office 365, never hybrid) and was working perfectly well up until a few weeks ago with no apparent changes to bring on this problem. Messages sent to an alias on this account in a different domain (the .onmicrosoft.com domain) go through fine, it's just messages to our primary domain that get bounced.
I'm completely at a loss to explain why this is happening.