I think that you have mailjet sending outright) outgoing mail for Discourse. This is for notifications and address verification.
If you want to handle mail sent to your domain name you can do it however you want. You could set up a mail server on you digital ocean droplet to receive that mail and do whatever you want, but since what you want it for it to be forwarded elsewhere, it’s not the easiest solution.
I would guess that mailjet would receive and forward the mail for you.