When searching for messages for a particular user, generally someone on the IT or messaging team, you may notice most, if not all, messages are not displayed in the portal. This can sometimes be caused by a misconfiguration of the Journal Rule in Exchange.
When configuring a journal rule in O365 you must pick an NDR recipient. This user should be a shared mailbox or service account that your peers have access to as it is a good first indicator there is an issue with the journaling service. A Non-Deliverable Report (NDR) is created for each message that fails to be journaled.
To ensure an NDR loop is not created, Microsoft does not journal messages sent to the NDR recipient. If a message sent to email@example.com was attempted to be journaled and fails, it would create an NDR email, which would then also fail to journal, creating another NDR, and then you have a never-ending loop.
To verify who your NDR recipient is, simply go to the Exchange Admin Center > Journal Rules and look for the hyperlinked email address at the top. The most common is to create a shared mailbox so others can also monitor any NDRs that may come in.