In Outlook 365, older emails that are not deleted and purged automatically get retained, according to our email retention policy, and stored in an online archive.
To make room in your archive mailbox, delete any items you don't need, including Junk Mail or older messages, and empty your Deleted Items folder.
Permanently erase messages from Outlook
Once you have deleted an email, it moves to a Recoverable items store. To permanently erase all of your Deleted Items you need to also purge all of your Recoverable items.
Warning: Items removed from your Recoverable items are permanently deleted and cannot be restored.
1. Under Folders, select Deleted Items.
2. At the top of the message list, select Recover items deleted from this folder.
3. Click the Select All button in the lower left to select all the emails or Ctrl+Click to highlight specific emails, then choose the radial Purge Selected Items (or Restore Selected Items if you wish to restore them to the Deleted Items folder) then select Ok.
Learn more about archive mailboxes and storage limits.