Applies to Exchange 2013
Some items cannot be deleted. They were either moved or already deleted, or access was denied
You get above error when trying to delete mailbox Items from Outlook in Non-Cached Mode
Run the below exchange shell script to check if the concerning Mailbox Recoverable Items Quota is reached
Get-MailboxFolderStatistics -Identity MailboxUser -FolderScope RecoverableItems | fl name,*size*
If yes, either increase the quota or delete the contents in the RecoverableItems folder based on criticality of the user
To delete the content; use the below Exchange Shell Script
Search-Mailbox -Identity MailboxUser -SearchDumpsterOnly -DeleteContent
Once done; you will be able to delete individual folder mail items