Mac Mail deleted an entired thread of emails without my consent
Don't know what your current settings are, But for future safety, in mail → preferences → accounts → mailbox behaviours, keep trash mails for a month. In case they were deleted by an inadvertent action. 

If there were a backup, ~/Library/Mail/Vx (where x is a number) would have the mails. You can try though.

You can also ask your IT department(if any) to check in their backups. Most of them keep it for some days.