A Domino server sends mail to the internet through a forwarding host (e.g., any sort of internet provider). For failover, the customer contracts with 2 or multiple internet providers (or runs two independant firewalls, whatever there is). If the forwarding host fails, there seems to be no automatic re-routing to the second forwarding host. The administrator needs to change for forwarding host entry manually. This may lead to substantial timeouts and delayed delivery of mail.
(first priority) In the Messaging Settings document, allow for multiple entries in the "forwarding host" field, and possibly (second priority) some configuration (round-robin, or failover only)