I've recently noticed one of the qmail SMTP servers I adminster had plenty of logged spammer emails originating from yahoo.com.tw destined to reache some random looking like emails (probably unexisting) again to *@yahoo.com.tw
The spam that is tried by the spammer is probably a bounce spam, since it seems there is no web-form or anything wrong with the qmail server that might be causing the spam troubles.
As a result some of the emails from the well configured qmail (holding SPF checks), having a correct existing MX, PTR record and even having configured Domain Keys (DKIM) started being marked, whether emails are sent to *@yahoo.com legit emails.
To deal with the shits, since we don't have any Taiwanese (tw) clients, I dediced to completely prohibit any emails destined to be sent via the mail server to *@yahoo.com.tw. This is done via /var/qmail/control/badmailto qmail control variable;
Here is content of /var/qmail/control/badmailto after banning outgoing emails to yahoo.com.tw;;;
qmail:~# cat /var/qmail/control/badmailto
The first 4 lines are default rules, which are solving a lot of badmailto common sent emails. Thanks God after a qmail restart:
qmail:~# qmailct restart
Checking in /var/log/qmail-sent/current, there are no more outgoing *@yahoo.com.tw destined emails. Problem solved …