This question has been bugging me since Clutter was launched, and I was happy to find this thread on the IT Pro Network that answered it. Clutter is one of those features that I take for granted now, but it’s definitely a question that comes up during migrations when users are starting to see it, and some aren’t (yet).
“Let me clarify the issue here:
Clutter is a learning system. It requires to have a certain lower limit of messages in the mailbox to confidently learn about a user’s behavior before Clutter is auto enabled for a mailbox.
For newly created mailboxes and mailboxes that are migrated from On-Prem to the cloud, we need the following requirements to be satisfied:
1) At least 1000 messages delivered to the mailbox after creation (or migration to the cloud).
2) User needs to have logged into the mailbox once after creation (or migration to the cloud).
After the above two criteria are satisfied, Clutter is auto enabled for that mailbox within 24 hours.”