Dual Static IP on SBS2008

I have a SBS2008 behind a CradlePoint dual WAN failover. The CradlePoint has a cable ISP as well as a cellular ISP hooked up and working in failover so that when cable goes down, it jumps over to cellular and back when the cable comes back up.

SBS2008 currently receives email through static IP address configured at cable ISP. When CradlePoint is in failover, client can send email, but not receive.

Is there a way to have email come into the server regardless of what ISP it's connected through? Client is already jumping to tell cellular ISP to provision static IP - but how do I use it?
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Elixir2Asked:
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James NewportCommented:
If the cellular ISP issue a static IP - which I doubt if they will issue, then you would create another NAT rule for the 2nd WAN IP address to pass through to the server.
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Elixir2Author Commented:
The 2nd WAN already passes through to the server and that NAT rule is already in place. The problem is higher up the pipe on the incoming side. I guess I should clarify the scenario. This is the link I've been basing my comments from. It says I'll have to change DNS and MX records to accommodate both IPs? I'm actually curious if this is even possible - i mean in practice rather than theory.

Here's the nutshell:
1. email flows through cable isp to server.
2. when cable goes down, failover to cellular occurs
3. on cellular, inbound mailflow stops b/c it can't find cellular path
4. client can still send out email in all situations

How do we prevent #3?
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James NewportCommented:
You'd need to put cellular static ip as a lower priority mx record.
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Elixir2Author Commented:
So, in practice, is this workable? I mean how long does it take for email to hit the higher priority MX record, fail, and retry? I have never actually seen this work in real life.
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James NewportCommented:
If it can't connect on the first mx record it will try the second. It works for me... I have multiple records set up if I switch off a server it is held by a backup mx provider
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Elixir2Author Commented:
Yep, I have the backup MX provider in place already. I suppose if I implement that dual IP scheme, I'll have to drop the backup MX even lower in pri, correct?
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James NewportCommented:
yes thats correct
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