**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:
As explained, Email client to Email server communication is not related to the MX records defined. MX records is used only for Email server to Email server communication.
Assume you had hosted your own email server behind the Balance 580
Public DNS server is hosted at ISP.
Email server to Email Server communication (Outgoing/Incoming emails):
DNS records defined for ISP hosted DNS server:
Primary mail server : mail1.ourcompany.com MX 10 on ISP1 connection
Backup mail server : mail2.ourcompany.com MX 10 on ISP2 connection
You only need to had the corresponding Port Forwarding Rules/NAT mapping rules for ISP1 & ISP2. Please refer to the details steps below for Port Forwarding Rules/NAT mapping rules.
Delegation:
mail NS WAN1BalanceWANIP
mail NS WAN2BalanceWANIP
WAN1BalanceWANIP WAN1_IP_Address_Balance580 <— IP address
WAN2BalanceWANIP WAN2_IP_Address_Balance580 <— IP address
Make sure DNS service is enabled for B580 & Domain Name: ourcompany.com is created for mail A records