Hi,

We're designing a new mail system which sits behind a load balancer.
One component of this system is a set of hosts dedicated to queuing
mail for domains to which we offer secondary MX. Our intent is to use
the load balancer to present a single routable IP address to the
outside world regardless of how many hosts reside behind it. The
queuing hosts are configured with non-routable IP addresses visible to
the load balancer only. Domains for which we offer secondary MX will
refer to the routable address in their MX records. When the primary
MX host is down, mail will wait on our queuing hosts until the primary
comes back up.

We'd like to use the load balancer front end instead of round-robin MX
records because concealing the queuing hosts behind the load balancer
allows us to remove hosts from service and add new hosts without
modifying DNS. This also simplifies DNS considerably, as we'll have a
single secondary MX record for all domains instead of one for every
queuing host. We currently provide secondary MX for over 1,000
domains, so this is a significant savings.

The network diagram, example DNS records, and an abbreviated Postfix
configuration for this system are as follows...
    

    ________________________________________
    NETWORK DIAGRAM

               Internet
                   |
              206.26.36.1 (mxqueue.easystreet.com)
                   |
           +---------------+ ROUTABLE NETWORK
    - - - -| Load balancer |- - - - - - - - - - - - - - - - - - - - - - -
           +---------------+ NON-ROUTABLE NETWORK
                   |
          +-----------------+
          | |
     192.168.0.1 192.168.0.2 (mxqueue01. and
mxqueue02.easystreet.com)
          | |
    +-----------+ +-----------+
    | mxqueue01 | | mxqueue02 |
    +-----------+ +-----------+

    ________________________________________
    EXAMPLE DNS RECORDS

    domain.com. 86400 IN MX 20 smtp.domain.com.
    domain.com. 86400 IN MX 80
mxqueue.easystreet.com
    mxqueue.easystreet.com. 86400 IN A 206.26.36.1

    ________________________________________
    ABBREVIATED POSTFIX MAIN.CF CONFIGURATION

    myhostname = mxqueue01.easystreet.com
    mydestination = $myhostname, mxqueue.easystreet.com
    inet_interfaces = $myhostname
    relay_domains = hash:$config_directory/relay_domains
    alias_maps = hash:$config_directory/aliases
    alias_database = $alias_maps

We're trying to determine the correct Postfix configuration for this
network design. In our experiments, mail sent to domain.com arrives
on one of the MX queuing hosts as expected if smtp.domain.com is down,
but Postfix immediately tries to redeliver it to
mxqueue.easystreet.com because it doesn't realize that it is acting as
mxqueue.easystreet.com. The result is a mail loop.

Our question is this: can Postfix be configured into thinking it
should queue mail as if it is mxqueue.easystreet.com despite the host
having no network interfaces configured with this name? Adding
mxqueue.easystreet.com to $mydestination has no apparent effect.

Any advice would be greatly appreciated. Thanks!

                 - - - - - -*- - - - - -
  
 - Kenneth Maupin EasyStreet Online Services,
Inc. -
 - Senior Systems Engineer Beaverton,
Oregon -
 - maupineasystreet.com
http://easystreet.com/ -
                 - - - - - -*- - - - - -

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