You appear to describe a system that delivers mail to two independent
message stores. The problem with that approach is that if a user
reads or deletes a message on one message store, then their action
will not change the state of the other message store.
It is possible to do better than that.
If you need a consistent view of multiple message stores, then you
need a replicated message store. Dovecot supports asynchronous
replication between a pair of master servers. You configure Postfix
to deliver mail to one of the two Dovecot instances (e.g., using
one DNS name with two A records, or random hostname selection with
the Postfix randmap feature), and the Dovecot instances talk to
each other. When one Dovecot instance is down, Postfix will deliver
to the instance that is available.
> I understand how a MX relay works and how to implement it in postfix, but
> what I am looking to do is create a clone of all the mailboxes on our
> system to another system. So is one server setup like a MX backup relay and
> then there is some switch or option that I don't know or do I just setup
> 'virtual_alias_maps' in main.cf on both systems to save a copy of the email
> and forward it to the other? Will postfix be smart enough not to get stuck
> in a loop?
Two-way forwarding between MTAs is the definition of a mail forwarding
loop. Postfix breaks loops by looking at the hopcount_limit and at
Delivered_To: headers, and that results in a non-delivery notification
to the sender. So don't do that.
Wietse
