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 (VPN IP
Secondary WAN 2 - (Email IP (VPN IP

If the Primary WAN 1 fails my Email Server and VPN connections to the ouside world are seen as
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: