Other than reducing the mailbox size which seems to be the usual advice, is there anything you can suggest to test connectivity or determine what is causing the Outlook freeze? It seems to affect users which have a shared mailbox delegated to them or access a mailbox in online only mode. While this group of users are the main ones affected there are others in the organisation that have freezes too and they aren't using the same secondary mailboxes. They recommend caching user mailboxes and un-caching shared mailboxes or at the least disabling download of shared folders – We already have the secondary mailbox in online only mode. The only suggestions from Outlook support were that users should be using a shared mailbox instead of a user mailbox but I cant see how this would make any difference at all to connectivity to ExO. Tweaking NIC settings to disable low power mode etc.Checking Outlook connection status (control right click on Outlook icon > Connection status) for errors (none) and reconnecting when locked up - No effect, doesn't brink Outlook back.Contacting Outlook support via Outlook Help menu (zero luck there).

Checking AutoDiscovery settings - Nothing obvious or critical identified from what I can see.

Running SARA/Outlook diagnostic tool – Nothing obvious or critical identified from what I can see.

Disabling “Optional connected experiences”.

Disabling graphics hardware acceleration.

Running continuous ping in PingPlotter to determine any connectivity issues (none identified).

Applying all windows updates and Office 365 updates.

Enabling cached mode for the secondary mailbox and setting retention to 3 months.

Disabling Anti-virus (Webroot SecureAnywhere).

Disabled web plugins when outside of Safe mode.

Running Outlook in Safe mode to eliminate plugins.

I have tried the following none of which has made a difference to performance with users : The mailbox is in online only mode because caching not only requires a huge OST and takes a long time to download on new installs but also results in delays between users when items are moved, flagged or categorised. This specific team uses the secondary mailbox to triage and manage many incoming emails with attachments and they need to coordinate with each other and move mail items into sub folders based on region or job type. Second user/mailbox is assigned E3 licence.

In Place backup is enabled for the second mailbox. Users are configured with personal user mailbox and secondary user mailbox (50Gb in size) which is not cached. We have multiple Outlook 365 users experiencing freezes/white screen from 10 seconds to 5 minutes or more when using a large shared user mailbox.

Outlook crashes when opening images