Interfaces and routes on iRedMail mail server?

Question

linux

Good afternoon!
Available: Debian mail server (iRedMail) with internal (LAN) and external (WAN) interfaces.
LAN IP: 192.168.0.16
WAN IP: 91.x.x.50
The server itself is virtual (VMWare ESXi hyper), in addition to all this, it has the ability to access the Internet through the LAN interface, because there is a gateway (pfSense 2.2.6) with IP 192.168.0.1 which distributes the Internet to LAN, the gateway, in addition to all this, has two Internet channels (one is the main one, the second is a reserve). So, the first provider (main) gives us three IPs (91.x.x.50, 91.x.x.51 and 91.x.x.52), now one of the addresses (91.xx50) goes directly to eth1 is the mail server interface (WAN), the second (91.x.x.51) goes to the pfSense gateway, the third (91.x.x.52) is in reserve.
Task: how to make the mail server continue to work when one of the providers crashes?
My options:
Option 1: you can remove the WAN interface from the mail server (91.х.х.50) and leave it behind NAT with only one local LAN interface 192.168.0.16, register all the necessary "redirects" of ports (25 , 110, 993, etc.) from two WANs to the same mailer (192.168.0.16) and then if one of the ISPs on the gateway fails, the server will be "online" and will catch/send mail;
Option 2: add a second static route (if possible) so that the server can "go online" through its eth1 (WAN) interface, and if it is down, then it can go out through the LAN interface (eth0). Is it possible?