There have been some discussions online where people are saying that Clutter seems to be disabled by default for new mailboxes in Exchange Online. It turns out that Clutter has not been disabled by default for new Office 365 mailboxes. As we know, Clutter is a learning system, and it requires 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. This means that, for newly created mailboxes and mailboxes that are migrated from an on-premises environment into Exchange Online, the following requirements need to be meet:
At least 1,000 messages delivered to the mailbox after the mailbox was created or migrated;
The user needs to have logged into the mailbox once after creation/migration.
After both these requirements are meet, Clutter is auto enabled for that mailbox within 24 hours.