Can we use Exchange Online Mailbox for Journaling purpose

Can we use Exchange Online Mailbox for Journaling purpose ? – NO

The journaling mailbox specified in a journal rule must be external to your organization (and External to Office 365. Cannot be in a different tenant).

Why it is not supported?

A journal mailbox often handles tremendous traffic. Imagine if you enable journaling for 1,000 users, each of which receives 50 messages daily and sends 25 responses. That means that the journal mailbox will handle a minimum of 75,000 messages daily. Now scale it up a tad to 10,000 users and you begin to realize just what a strain the mailbox comes under. An Exchange mailbox can absolutely handle the demand, but you need to prepare for the load. For example, a journal mailbox might be assigned its own database and placed on a separate volume to ensure that it never exceeds quota or available disk storage. That kind of custom mailbox doesn’t exist in the pre-canned world of cloud systems.

Journaling service providers have infrastructure dedicated for handling Journaling emails, requests and dealing with Customer requirements when emails are required for any legal matter.

Even the users archiving mailbox is only intended for that particular user.

Using journaling, transport rules, or auto-forwarding rules to copy messages to Exchange Online Archiving for the purposes of archiving is not permitted. A user’s archive mailbox is intended for just that user. Microsoft reserves the right to deny unlimited archiving in instances where a user’s archive mailbox is used to store archive data for other users.