In http://www.postfix.org/STANDARD_CONFIGURATION_README.html this text ...
A null client is a machine that can only send mail. It receives no
mail from the network, and it does not deliver any mail locally. A
null client typically uses POP, IMAP or NFS for mailbox access.
... is confusing (the part about POP, IMAP, or NFS). Can this
configuration be used for a null server (I'm guessing this term)?
This new server only needs to send mail. It has no access to the
internet (though it does have access to the internal DNS caches and
the real mail server via private IPs) so it won't be able to reach the
MX host for recipients by means of the IP address it would get via MX
and A lookups. It just needs to always forward everything via the
primary mail server (or the secondary if not configured to have a send
queue of its own). It won't receive or read any email (no POP, no
IMAP, no NFS).
--
sHiFt HaPpEnS!
