Failover for incoming connections

I am researching failover options for my company. I am looking into the Peplink 210. I am curious to how the PepLink deals with my external users.

Example: I have an Email Server and VPN

Primary WAN 1 - (Email IP 99.99.0.1) (VPN IP 99.9.0.2)
Secondary WAN 2 - (Email IP 77.77.0.1) (VPN IP 77.77.0.2)

If the Primary WAN 1 fails my Email Server and VPN connections to the ouside world are seen as 99.99.0.1-2
After the swap how does the PepLink ensure that the end user still sees 99.99.0.X despite the fact that it has failed over to 77.77.0.X ?

This can be accomplished using the Authoritative DNS feature, more details:

Thanks.