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) and/or prohibition by either MTA,
 do the following:
   1) # nslookup -type=mx 'domain.tld'
        domain.tld     mail exchanger = 0 mx.domain.tld.
   2) # mkdir /var/qmail/control/notlshosts/
   3) # touch /var/qmail/control/notlshosts/mx.domain.tld

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

 No encryption for any domain except for those specified can also be configured.

 Complete information on the subject can be had by typing '# man qmail-remote' at the CLI, 
 or by looking here.