The Delete button is disabled in Outlook's Inbox

Several users noticed the Delete button was missing when a message was selected in the Inbox:

I recently switched email accounts and the Delete button is disabled and some of my messages are missing. The Delete key on the keyboard will delete messages, only the Delete button in the Home ribbon and the right-click menu for selected e-mail(s) is disabled.

This is the result of exporting an IMAP data file to a pst file or opening IMAP pst files in Outlook. The folders retain certain IMAP features, one of which is the Delete button doesn't work as expected. (In IMAP, it purges.) To fix it, you need to edit the Inbox folder using MFCMAPI and change the PR_CONTAINER_CLASS to IPF.Note.

Get MFCMAPI . If you have 64 bit office, you need 64 bit MFCMAPI.
Open MFCMAPI, go to Session, Logon
Select your profile
Double-click on the affected data store.
Expand Root - Mailbox
Expand Top of Information Store or IPM_Subtree
Select Inbox
Look for PR_CONTAINER_CLASS in the right pane, double-click on it.
Change IPF.IMAP to IPF.Note then click OK to close the dialog.
Repeat for each folder that was exported from an IMAP data file.
When you are finished, close the MFCMAPI window, click Session, Logoff on the last MFCMAPI window.