Recover deleted email trash:
"I tried on my computer too. The emails have been wiped clean"
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A. Once Deletion is Complete - it's Complete:
If it were thousands, then I'm certain it would take a while just to delete them all. So, maybe they have successfully been deleted?
B. Contact the eMail Provider:
And this is iCloud, correct? If it is some other eMail provider (i.e. Google, Yahoo, or Microsoft), then contact them.
C. Any Time Machine Backups?:
I. Recover eMails from a Time Machine Backup:
If you set up Time Machine to back up files on your Mac, you can use the backups to restore lost mailboxes and email messages. See Restore items backed up with Time Machine.
II. Restore Mac from a Time Machine Backup:
If there are any Time Machine Backups, then restore your Mac to that backup. From there, keep the network connection(s) disabled. Nothing would sync, being no connection are made. Once stored, export them to an external hard drive. Once exported, create a new folder on the eMail account, and import them to that new folder. Go Here: Import or export mailboxes in Mail on Mac - Apple Support