Missing deleted email messages in Trash folder

FrustratedinSCCA wrote:

I suddenly can't see any email messages I've deleted using the trashcan icon (delete) in my mail's Trash folder. It only works if I drag a message to the Trash folder. I have checked the box in "mailbox behaviors" to send discarded messages to Trash.

You can try quitting Mail.app Relaunch the app holding the Shift key down—compare you results.

For problem accounts the go to way to sort things out is to delete the account(s) from the Mail.app, then add it back.

Remove email accounts in Mail on Mac

Remove email accounts in Mail on Mac - Apple Support

Add email accounts in Mail on Mac - Apple Support

Add email accounts in Mail on Mac - Apple Support

Note—

IMAP accounts stay on the server, so no local loss, when you re-sync

POP accounts download and remove from the server—therefor save your inbox/sent box if you want them

“On My Mac" is just that, local folders you created and moved mail into. So no loss