ZCS 8.5 or above onwards this attribute is now in ldap - zimbraMtaLmtpHostLookup

zmprov ms mtaserver.com zimbraMtaLmtpHostLookup native

In case that you are using Single Server, be aware always of the Global Config as well:

zmprov mcf zimbraMtaLmtpHostLookup native

Once this is done, you'll need to restart the mta:

zmmtactl restart