One possibility, one or more domain inside your server send a spam. Remember, it's possible php send email via SMTP with 'unknown' identity (aka domain) but qmail-toaster permit because from inside (aka localhost). You can try modified tcp.smtp:
from:
127.:allow,RELAYCLIENT="",DKSIGN="/var/qmail/control/domainkeys/%/private"
:allow,BADMIMETYPE="",BADLOADERTYPE="M",CHKUSER_RCPTLIMIT="50",CHKUSER_WRONGRCPTLIMIT="10",DKSIGN="/var/qmail/control/domainkeys/%/private"
to:
127.:allow,BADMIMETYPE="",BADLOADERTYPE="M",CHKUSER_RCPTLIMIT="50",CHKUSER_WRONGRCPTLIMIT="10",DKSIGN="/var/qmail/control/domainkeys/%/private"
:allow,BADMIMETYPE="",BADLOADERTYPE="M",CHKUSER_RCPTLIMIT="50",CHKUSER_WRONGRCPTLIMIT="10",DKSIGN="/var/qmail/control/domainkeys/%/private"
With this trick, qmail will process all smtp (inside or outside) with the same rule.