Mailbox performance issues after migrating user to 365

Question

In the process of migrating mailboxes to 365 from Exchange 2016 and having major issues with one particular user. User is in cached mode and we've had to rebuild the profile and OST and it went extremely slow but eventually finished. We have the slider set to 3 months but Outlook constantly shows Updating in the lower right and freezes every time he switches between folders, sometimes requiring a restart.
User has one folder that is 39 GB and contains over 330,000 items going back one year. This is in his primary mailbox. He also has an online archive. Tried using a policy tag on that folder to delete 6 months and ran managed folder assistant but nothing is deleting.
The problem is severely impacting this user's productivity and he is asking to be moved back to Exchange on-premise but that will likely take days because of the size.

Any suggestions on how we can make this better? I know that folder is too big but don't know of a good way to delete that many items efficiently other than with the policy tag. Is there a utility that would let us save that folder to PST's without having to use Outlook?

Thanks in advance.

Answer 1

Do you want to delete the items/move to the online archive or export to a pst? If pst, then there isnt much in the way of options other than what you have seen.
If by policy and its not working as you have said, then note its never immediate in 365. It will take some time for the Assistant to move/delete the items per your policy. In fact there is a 7 day cycle and from what I have seen, you arent gonna make that go any faster running that command :)

https://learn.microsoft.com/en-us/office365/troubleshoot/archive-mailboxes/message-not-moved-to-archive

The retention policy runs automatically one time every seven days for mailboxes that are larger than 10 MB.

Answer 2

Hi @Kenny Stern

Yes, as oscarmh suggested above, we can use compliance search in o365 to meet this need deleting messages through powershell.

Here is the official document for your reference: Search for and delete email messages

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Answer 3

Looks like compliance should do that you need a search pattern, the cmdlet is New-ComplianceSearchAction -SearchName Phish1 -Purge -PurgeType SoftDelete

https://adamtheautomator.com/office-365-delete-email/

Answer 4

Yes indeed !! Have you seen this https://learn.microsoft.com/en-gb/microsoft-365/compliance/use-network-upload-to-import-pst-files?view=o365-worldwide ?? Hope it helps !!