To stop TLS encryption for a particular domain as QMT will encrypt via TLS all SMTP traffic unless 
   1) A remote MTA does not support it
   2) Incompatible encryption between MTA(s)
   3) Prohibition by either MTA
     a) # nslookup -type=mx 'domain.tld'
        domain.tld     mail exchanger = 0 mx.domain.tld.
     b) mkdir /var/qmail/control/notlshosts/
     c) touch /var/qmail/control/notlshosts/mx.domain.tld

     This will stop all traffic to domain.tld from being encrypted.