Hi @Pascal M ,
In case the issue occurs because the public folder mailbox that hosts the public folder have reached its recoverable items quota, I'd recommend following the steps below to release some storage of the Recoverable items folder and check the result:
- Connect to the public folder by using Outlook desktop client.
- Go to the public folder, and select Recover deleted items on the ribbon.
- Select items to delete, select Purge Selected Items, and then select OK.
- Try deleting mails again.
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