This specification adds the 556 reply code to the repertoire specified in SMTP. When an intermediate SMTP system (typically a relay) that would normally attempt to open a mail connection to a host referred to in a forward-pointing address can determine that the host does not accept mail connections, and do so without attempting to open a connection to that target host, it is appropriate for it to return a reply with a 556 code to the system that sent it the message for outbound transmission. Interpretation of a special DNS record, found when a lookup is performed in conjunction with a RCPT command, is one such method (and the only standardized one at the time this specification was written).
When an SMTP server returns a 556 reply code after receiving a command (such as RCPT, which contains a forward-pointing address) because it has information (such as discussed above) that the mail will not be accepted, the SMTP client is expected to handle the response like any other permanent negative completion reply to the command. This is consistent with the SMTP specification.