Problem: around 30 users have the same problem. They are saying that they are not getting emails from "email@example.com", but according to message trace all emails are successfully delivered to their mailboxes.
I've advised users to check the following:
*focused inbox - no result;
*check OWA if emails are showing - they do not;
*rules/policy - there are none;
*Outlook search can't find email;
*email address "firstname.lastname@example.org" is whitelisted;
*Delete emails/Junk folder/archive folder - no result;
The detailed message trace will also tell you to which folder the mail was redirected, in case any rule acted on it, so check for that. In addition, this might be an instance of a message being ZAP-ed post-delivery, as the protection layer has re-evaluated it as spam/malware. Again something that the trace should show.
As a last resort, you can also run an eDiscovery search or good old Search-Mailbox against the mailboxes in question.