The qmail Handbook

qmail Doesn't Use /etc/hosts

qmail never uses /etc/hosts to determine the Internet Protocol (IP) address associated with a host name. If you use names in control files, qmail must have access to a name server.

It is possible to run qmail on systems without access to a name server. Hosts in control files can be specified by IP address by enclosing them in square brackets ([]), for example:

[10.1.2.219]

Actually, the square brackets aren't always necessary-but it's a good idea to use them anyway.