I'm testing it right now. And now it works just like I want.
When I put
sales@domain.com in the canonical address (user account properties in the admin interface) and John Doe sends out an e-mail return-path gives:
sales@domain.com. When I remove the canonical address return-path gives
joh@domain.com
So it seems to work fine for mine situation now.
The only problem you can face with this solution is that when you use different persona with different e-mail addresses, that will not work anymore.This is because it always uses the e-mail address used in the canonical address. So keep that in mind when you make use of the canonical address field!