Exchange Online "mailbox full" but the online archive isn't helping - a read-first troubleshooting guide

This troubleshooting guide outlines the steps to resolve Exchange Online mailbox issues when it appears full, despite the online archive being enabled. To begin, confirm what is actually full by checking the primary mailbox, Recoverable Items folder, or Outlook's local data file. Use the provided commands to obtain quota information for the user's mailbox.

Next, verify that the archive is indeed enabled by examining the user's mailbox properties. Check for a non-zero ArchiveGuid and a non-empty ArchiveName. If an archive exists, also examine its size to determine if anything is moving mail to the archive.

The most common reason for a full mailbox is a retention policy with a Move to Archive tag, processed by the Managed Folder Assistant. To address this, review the user's retention policy and its tags to ensure they are set up correctly. If the policy only moves old mail and the user has recently sent or received emails, the archive will remain unused. Make sure retention processing is enabled for the mailbox and that the Managed Folder Assistant is functioning as expected.

Finally, if the Recoverable Items folder is near its quota, examine its contents to determine if the issue lies there instead. By following these steps in order, you can quickly identify the cause of the mailbox full issue and take appropriate action to resolve it.