Dear Tara B,
Good day! Thank you for posting in the Microsoft Community.
To help us narrow down the issue you are facing kindly check the troubleshooting steps below:
Check for retention policies:
- In the Outlook on the Web > settings > Navigate to Mail > Retention policy.
- Review the policies and ensure that they are set to permanently delete items from the Deleted Items folder.
See the screenshot for reference:

Use PowerShell to Delete Items**
If you have administrator access, you can use PowerShell to clear the Deleted Items folder. If no ask the admin to do it for you. Here are the steps:
- Open PowerShell as an administrator.
- Connect to Exchange Online using:
Connect-ExchangeOnline -UserPrincipalName your_email@example.com
- Run the following command to clear the Deleted Items folder:
Search-Mailbox -Identity "your_email@example.com" -SearchQuery "Folder:DeletedItems" -DeleteContent
If the issue persists kindly reach out to your admin to open a ticket in the Microsoft 365 admin center and the technical support team will help you check this issue from the back end. Get support - Microsoft 365 admin | Microsoft Learn
We are looking forward to your reply. I appreciate your reply.
Sincerely,
Microsoft Community Moderator