Proofpoint / SMTP error code 550
smtp;550 5.1.1 <email@example.com>: Recipient address rejected: undeliverable address: host example.com[x.xx.xx.xx] said: 550 5.1.1 <firstname.lastname@example.org>: Recipient address rejected: user unknown
Mail is routed through mailservers owned by Proofpoint before it's passed along to the recipient domain's mailservers for final delivery. If the message is rejected when Proofpoint attempts to finally deliver the message, they'll quote this rejection within the bounce response they send back to you. This is an example of that kind of relayed/quoted bounce.