I have a user that has complained she sometimes doesn't receive an email from a few people into her inbox.
Their office 365 is configured so that if an email is sent to them from an external domain, it is BCC'd into a specific mailbox as a 'catch all'. She can see the email in the BCC mailbox addressed to her but not in her own inbox. Other users that are addressed in the email are receiving a copy in their own inbox. This is intermittent as she can receive other emails send from these people so it should rule out blocked sender and spam filter.
I have completed a message trace on the O365 tenant and it says 'delivered'. I have searched her mailbox on OWA and the specific message is not there. I have checked blocked senders and the addresses are not listed (I've added them to safe senders as a precaution)