smtp_skip_5xx_greeting
yes
When an SMTP server responds with 5xx reply code, Postfix can
either bounce the message or move on to any additional mail
exchangers for the destination domain to see if they are able to
accept the message. The parameter smtp_skip_5xx_greeting specifies whether or not Postfix should react to the
reply code or move on. A value of no causes Postfix to try additional mail
exchangers.