[PLUG] How does having a secondary MX work?

[Stafford A. Rau]

* Steven Raymond <stever at woo-hoo.com> [020914 21:33]:
> 2)  Must you have all the existing user accounts receiving mail on primary
> MX server also exist on the seconary MX server?  In my testing, sending
> mail to existing-user at primarymx if they do not have a local user account
> on secondary MX server results in bounced with "user unknown".

Sounds like your secondary mx server is configured as final destination
for your domain, rather than as a secondary mx.

If you want to set up "mail2.woo-hoo.com" as a secondary mx for the
"woo-hoo.com" domain, then in your postfix main.cf:

mydestination = $myhostname, localhost
# Note - don't include $mydomain or woo-hoo.com in mydestination!

relay_domains = $mydestination, woo-hoo.com

This says you'll rely mail for woo-hoo.com, but you're not the final
destination for it.

When your primary is down, mail for woo-hoo.com will queue up on the
secondary, but won't be delivered to local users in the mail spool file
or whatever (unless it's addressed to user at mail2.woo-hoo.com).

The mail will stay in the deferred queue on the secondary, and it will
keep trying to relay it on to your primary until the primary comes back
up and it can deliver it. Note that the longer your primary is down, the
longer the secondary will wait between attempts to deliver it up to a
configurable maximum, so don't get nervous when it doesn't deliver it
immediately when the primary comes back up.

You don't need to duplicate accounts on the secondary from the primary
this way.

--Stafford