I've got a similar setup with redundant internet connections with 2 static IPs.
My external DNS mx records are setup as per post #7, an mx record pointing to 'mail.domain.com' and a second mx record pointing to 'mail2.domain.com', and a A record pointing to each of the different static IP addresses.
Though I can ping both IP addresses, zimbra only responds to the first record which has a matching hostname. Does the fact that the second MX record points to 'mail2' when the zimbra server has a hostname of mail cause an issue. If so, is it possible for the zimbra server 'mail' to acknowledge the 'mail2' alias?