Sites that have a need for multiple mail exchangers may enjoy
the convenience of using a networked mailer database, but do not
want to introduce a single point of failure to their system.
For this reason we've included the ability to have Postfix
reference multiple hosts for access to a single pgsql map. This
will work if sites set up mirrored pgsql databases on two or more
hosts.
Whenever queries fail with an error at one host, the rest of
the hosts will be tried in random order. If no pgsql server hosts
are reachable, then mail will be deferred until at least one of
those hosts is reachable.
