SMTP blacklisted email response code

Question

I am implementing a blacklist filter for a SMTP relay, which allows the user to define rules based on sender email, receiver email, sender ip or a combination of these. What SMTP status code should it reply if the email gets blocked because it is found to be blacklisted?

Answer 1

Postfix defaults to handle reject_rbl_client blacklists by sending a 554 (Transaction failed) together with a textual explanation. A google search also suggest that being a rather common practice in dealing with blacklists.