This is actually good point, because I’m using HTTP API as well! Thanks!
I just tried it, and not sure if that even works, are you saying if I set envelope_sender to some email address, the bounce will be redirected to that address?
Yes, the rfc5321.from address is where a receiving MTA is supposed to deliver a DSN to if it accepts a message and then no longer is able to deliver it. RFC 5321 - Simple Mail Transfer Protocol
KumoMTA does not generate DSN messages out of the box, it can accept them and process them and then forward them if the MX record for the domain in the rfc5321.from points to KumoMTA. If you point it to the customer’s MX then KumoMTA will never see it.
@dapper-yeti yepp yepp, please don’t forget to setup SPF, DMARC and stuff so that the emails won’t be rejected.