Once upon a time, Steve Searle <steve(a)stevesearle.co.uk&gt; said: Basically, you configure the secondary to relay (but not deliver) mail for the domain(s). For sendmail, this would mean putting a line like: To:example.com RELAY in /etc/mail/access (and running "make" in /etc/mail to update the db). When mail for that domain comes in, sendmail will see the MX record and attempt to pass mail on to a higher-preference (lower number) MX. However, there's a big problem with doing this (not a sendmail specific problem): the secondary doesn't know which @example.com addresses are valid and invalid. By default, it will accept email for all such addresses and try to forward them. When somebody sends an email to an invalid @example.com address via the secondary, the primary will reject the message and the secondary will generate a bounce message back to the apparent sender. This is a problem because spammers know about this loophole and will try to dump spam (to massive numbers of invalid addresses) on your secondary server (usually with forged sender addresses). The secondary will then bounce the spam to people that didn't actually send the messages; this is called "blowback" and will get your secondary server on spam blocklists in short order. You really need the secondary to have some way of knowing all the valid recipient addresses at the domain (and have any spam filtering configured to match), so it doesn't accept mail that the primary wouldn't. This is more complicated; for sendmail, you have to write a few custom rulesets (not really very much). The bigger issue is that you need some way for the secondary to know the valid addresses on the primary; the usual way is to have all users, aliases, etc. in LDAP (and replicate the LDAP to the secondary). -- Chris Adams <cmadams(a)hiwaay.net&gt; Systems and Network Administrator - HiWAAY Internet Services I don't speak for anybody but myself - that's enough trouble.