Although you received a bounce message indicating the email was not delivered (and rejected with a 5xx error at SMTP level) , your email was actually NOT discarded! The antispam provider of the recipient safely stored a copy of your email in the spam quarantine system of the recipient.
If your message was wrongly blocked, please reach out via alternate means to the recipient and request that they release and train the message from their spam quarantine, and then request a delist. This will not only resolve the immediate reputation issue, but also helps in improving your IP/domain reputation for the future.