Strange Outlook issue (Inbox of secondary mailbox doesn’t update)

Strange Outlook issue (Inbox of secondary mailbox doesn’t update)

This issue was escalated to me as it has bugged tech support and the affected users for a while.


  • Email System: Office 365/Exchange Online
  • Outlook version: Office 2016  (via O365 subscription)


UserA has been given full access to UserB’s mailbox (via Exchange Admin Centre – Recipients – User Properties – Mailbox Delegation – Full Access).

UserA can see UserB’s emails (Inbox and subfolders) in Outlook. However, UserB’s Inbox doesn’t automatically update. The workaround is for UserA to keep going to Send/Receive tab and click Update Folder which triggers a refresh and arrival of new emails in UserB’s Inbox.

The reverse doesn’t seem to happen. All settings being similar, UserB can see UserA’s emails in Outlook and it automatically updates with UserA’s new emails.

What has been done:

  • Permissions checked. Full permission to access each other.
  • Windows and Office patches all up to date.
  • Behavior when using OWA is ok. Which made us think it is an Outlook issue.
  • Outlook profile of UserA has been re-created.
  • Office/Outlook in UserA has been uninstalled then reinstalled.
  • Both users have deleted a lot of emails and shrunk their mailbox sizes.


This is a documented issue when the secondary mailbox contains a LOT of email folders (Note: folders and NOT messages). In the case of UserB, she had 986. The command to find this via Powershell is below.

(Get-MailboxFolderStatistics UserB).count


Reduce the number of mail folders (preferably to less than 500).


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