By default, Postfix sends address verification probe messages
via the same route as regular mail, because that normally produces
the most accurate result. It's no good to verify a local address
by connecting to your own SMTP port; that just triggers all kinds
of mailer loop alarms. The same is true for any destination that
your machine is best MX host for: hidden domains, virtual domains,
etc.
However, some sites have a complex infrastructure where mail
is not sent directly to the Internet, but is instead given to an
intermediate
relayhost. This is a problem for address verification,
because remote Internet addresses can be verified only when Postfix
can access remote destinations directly.
For this reason, Postfix allows you to override the routing
parameters when it delivers an address verification probe message.
First, the
address_verify_relayhost parameter allows you to
override the
relayhost setting, and the
address_verify_transport_maps
parameter allows you to override the
transport_maps setting.
The
address_verify_sender_dependent_relayhost_maps parameter
does the same for sender-dependent
relayhost selection.
Second, each address class is given its own address verification
version of the message delivery transport, as shown in the table
below. Address classes are defined in the
ADDRESS_CLASS_README
file.
By default, the parameters that control delivery of address
probes have the same value as the parameters that control normal
mail delivery.
