**Email server to Email Server communication: **
By default email application (servers) is designed to be able to connect multiple mail exchange servers base on MX records preference. So inbound load-balance/fail-over is depend the defined MX records for the DNS server.
MX1: ISP1 Public IP address
MX2: ISP2 Public IP address
What you need to do is make sure you have the Port forwarding or NAT mapping rules (multiple WANs) for the Inbound access and you shouldn’t have issue if ISP1 or ISP2 is down.
The only thing that you needs should be the inbound load balance or fail-over for the mail client (outlook) to mail server whereby A records is used. There are two method that you can choose for the inbound loadbalancing or fail-over methods. For more information please refer to the URL below:
Method 1 (Peplink as public DNS server for your domain):
DNS records configured within Peplink Balance
External DNS records at an Authoritative DNS Server