Correct. I don't want to change it server-wide, just for mail sent from Blesta.
After some more digging, it looks like Blesta uses Swift Mailer for email. According to the Swift Mailer docs, it's possible to set a separate return-path, but I couldn't find the identical code in Blesta's implementation. Unfortunately, I don't know enough about coding to recognize variations, so I'm not sure what change, or where to change it.