It ain't pretty, or recommended for the long term, but something like this should slow it down
/etc/postfix/main.cf header_checks = pcre:/etc/postfix/header_checks.pcre /etc/postfix/header_checks.pcre IF /^(To|From|Cc|Reply-To|Sender|Return-Path): / /@qq\.com/i REJECT ENDIF (check the syntax!)