> I'd better be using a couple of ASSP 2.0 sharing a common 

rethinking at the setup...

let me make an example with one domain; let's say we 
have the example.com domain; now, we will setup four
ASSP boxes at IPs 1.2.3.25, 2.3.4.25, 5.6.7.25, 6.7.8.25
and the public example.com DNS zone will read

@      IN MX 10 mx1.example.com.
@      IN MX 10 mx2.example.com.
@      IN MX 20 mx3.example.com.
@      IN MX 20 mx4.example.com.

mx1    IN A     1.2.3.25
mx2    IN A     2.3.4.25
mx3    IN A     5.6.7.25
mx4    IN A     6.7.8.25

@      IN TXT   "v=spf1 mx -all"

this way, we'll have two couples of "balanced" ASSP
working as primary and secondary MX for the domain
also, the TXT record will tell that only those servers
will be allowed to send email on the behalf of our
domain

the four ASSP boxes (note: I'm writing "boxes" but
they may as well be virtual machines btw) will then
use a fifth backend box hosting a MySQL database
and an OpenLDAP server used to check for valid
local mailboxes (the LDAP will be synced with the
backend mailserver or LDAP server) this box will
also run a couple of scheduled jobs; one to rebuild 
the spamdb on the "master" box (e.g. using ssh or 
the like) and then sync it on the other boxes; and
another one to download the clamD signatures 
and update them on the ASSP boxes so avoiding 
multiple downloads

all the boxes will then forward incoming email to a
locally installed SMTP daemon (sendmail or any
other critter will do); this, in turn, will take care or
routing the incoming mail to the backend mailserver

Outbound email will flow from the backend email
server to a "forwarder" which ... will be represented
by *all* the ASSP boxes; to do so, we'll setup the
DNS used by the backend mailserver so that we'll
have something like

assp    IN A     1.2.3.25
assp    IN A     2.3.4.25
assp    IN A     5.6.7.25
assp    IN A     6.7.8.25

so, setting up DNS round-robin and pointing the
backend mailserver to "assp.example.com" as the
smarthost, the outbound email will be handled by
whatever ASSP box in a "round robin" fashion, so
distributing the load amongst the four boxes; the
outbound email will again be forwarded to the ASSP
local SMTP which will in turn route it to the internet

The mail flow for such a config will be the following

* Inbound
sender MTA -> ASSP -> local SMTP -> backend mailserver

* outbound
backend mailserver -> ASSP -> local SMTP -> recipient MX

notice that, to keep things simple, I just wrote about a single
MySQL box and a single backend mailserver, btw nothing
forbids from setting up "clusters" for those two to avoid having
a single point of failure and/or adopting further "fault tolerance"
countermeasures

HTH


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