I use more accounts and all these account have a trash folder. There is also trash folder named On my Mac. Through the settings in my accounts the trash folders will be deleted after 2 weeks. However I cannot find where the trash folder On my Mac may arranged to delete the messages. The question is where this trash folder has to be arranged the same as the trash folders of my accounts.
I don’t think Mail has a setting for automatically deleting messages in the Trash mailbox On My Mac. However, if all your mailboxes are on the server, and you configure Mail to use the server Trash mailboxes, you shouldn’t end up with any deleted messages On My Mac.