Purchased a Vigor Draytek 2925n dual wan router.
Domain is hosted by dreamhost.com and the single mx record points to exchange.mydomain.com
Internet is provided by my managed office building: they provide a static IP address.
I'd like to get another ISP for the dual wan router for email redundancy, the second ISP can give me a static IP address but I am uncertain about what I need to update at dreamhost.
Setting aside the product or service recommendation (which is off-topic for the site), just add a second A record for the static IP address your secondary ISP provides and an MX record referring to that "A" record. The priority on that secondary MX will need to be based on the behavior of your router. I am not familiar with the Draytek router products. If the secondary connection can be active simultaneously with the primary connection then you might consider setting equal priorities on the MX records to split the traffic between the connections. If the secondary connection can only be active when the primary has failed then set a higher priority for the MX record that refers to the A record that refers to the secondary connection.
Exchange won't "care" about any of this. As long as the SMTP arrives at TCP port 25 of the Exchange server it's "happy".
