Failover ISP and email delivery

Client has a failover device where when one Internet connection goes down, it flips to the other, and back again when available.

Client wants email to be delivered to in-house server regardless of which connection he's on.

Can I just add a few MX records in the DNS for the secondary connection's IP address? Or will that screw with email flow? Reply if you've done this before.
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Elixir2Asked:
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John EastonDirectorCommented:
I assume here the client has two independent lines to his internal server - i.e. a leased line and ADSL backup or something similar.

I so, then setting up two MX records will do what you want.  If both connections are equal - i.e. two ADSL or two equal speed leased lines, then the MX priority can be the same.  If one is a backup give it a higher priority (i.e. first one priority 10, backup 20 etc).

One catch to watch out for is outgoing mail.  If you have an SPF record make sure both IPs are listed, otherwise mail sent from the backup could be marked as spam (as it is not being sent from your 'official' IP address).

Finally, please note some misconfigured e-mail servers could use the backup MX even when the main connection is up again, and indeed some may ignore the primary connection once it has gone down - even once it is backup again - as they can cache IP address.

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Elixir2Author Commented:
Very complete answer! Thank you.

In answer to the part I left unclear, he's actually using a Cradlepoint which has a WAN connection to Time Warner Cable (which drops 2-3 times per week) and the failover is a Verizon LTE modem. Client is in a unique location with only these options.

I especially appreciate your reminder about the SPF record, as we have already seen the spam-bounce on outgoing Verizon emails.
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