The MySQL configuration interface setup allows for multiple
mysql databases: you can use one for a virtual table, one for an
access table, and one for an aliases table if you want.
Since 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,
we've included the ability to have Postfix reference multiple hosts
for access to a single mysql map. This will work if sites set up
mirrored mysql 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 mysql server hosts are reachable, then mail
will be deferred until at least one of those hosts is reachable.
