How to purge emials from Deleted items in Microsoft 365 group?

Question

Answer 1

You have few options here.

1) Assign a retention policy that will automatically remove old messages: https://learn.microsoft.com/en-us/microsoft-365/compliance/create-retention-policies?view=o365-worldwide
2) Use the Get-RecoverableItems/Remove-RecoverableItems cmdlets. Probably not the best choice given the amount of items you have in the folder.
3) Use an EWS/Graph API based script to clean up the content of the folder. Similarly, you can use a low-level tool such as EWS Editor.
4) Open the Group in OWA and remove messages therein (right-click on the Folders node > Add shared mailbox > enter the group address).

Retention policy is probably your best choice here, as this will likely not be a one-time operation, and you might also have other groups needing the same treatment.