fault tolerant email?

[lgu at pobox.com (Larry Underhill)]

Hi folks, 

My group has recently setup our own email infrastructure for a project
we are running. Mail delivery and access via mutt is fine, but we are
now looking at sane strategies for fault tolerance. Anybody have any
pointers to resources or hard-won experience they would like to share?

Some details:

* Postfix + Courier IMAP on Debian
* We have two servers in different datacenters. Server "A" is the one
running mail services now. Server "B" is the one slated to provide
redundancy/fail-over capabilities. 
* We have regular backups of Server "A" to tape.
* Updating MX records in DNS is slow since we don't admin the
authoritative DNS server. 

We're most interested in solving the problem of continuing to receive
mail in the face of (a) serious hardware failure on the primary mail
server (b) serious network failure to the datacenter the primary mail
server is in. 

Any advice is truly appreciated!

--Larry