Does pop3/smtp transparent proxy exist on windows server?

Question

SMTP

Initial data:
An organization with 4 branches in different cities. 2 mail servers: master+backup are also physically located in different cities. Synchronization between them is configured. Scripts have also been written to switch them to different modes if necessary (physical failure of the master, Internet failure, etc.)
Task:
The address of the master server is registered in the mail clients. If it fails, then you will have to run to everyone and rewrite the backup server with your hands. Is there a free solution for a transparent proxy, so that clients can register its address, and on it it can already steer redirection to the desired server, or am I trying to reinvent the wheel and just NAT is enough?

Answer 1

What about DNS switching? By setting a pre-small record lifetime in the local DNS server. So, when switching, you don’t need to do anything at all and the traffic will flow correctly without any fierce proxying and changing port forwarding.