An extend on the last messages. For now i made the following changes.

Master.cf:
main     unix  -       -       n       -       -       smtp
  -o smtp_bind_address=192.168.1.50 
  -o inet_interfaces=192.168.1.50
  -o myhostname=mail.xxxxxxxx.yy
  -o smtp_helo_name=mail.xxxxxxxx.yy
  -o syslog_name=relay1
secundaire     unix  -       -       n       -       -       smtp
  -o smtp_bind_address=192.168.1.51
  -o inet_interfaces=192.168.1.51
  -o myhostname=failover.xxxxxxxx.yy
  -o smtp_helo_name=failover.xxxxxxxx.yy
  -o syslog_name=relay2

main.cf
sender_dependent_default_transport_maps =
  randmap:{main,secundaire}
smtp_connection_cache_on_demand=no

In this case the IP 192.168.1.50 and 192.168.1.51 are local IP addresses which PREROUTING and POSTROUTING on the main server to its real IP.

Is this the right configuration? Any suggestions?

Also is it possible to create a priority list instead of a randmap in "sender_dependent_default_transport_maps"

Hi,

IP failover are designed to be used on 2 different servers, not on the same one.

The principle is easy: you have 2 identical mail servers running syncing their data between each other (using rsync for example).

You then setup your domain DNS to declare 2 MX (mail) servers.

When a mail is sent to your domain, it will be redirected to the first MX server in your domain DNS record, which is your main mail server.

If for some reason your main mail server in unavailable, your domain DNS will route the request to the backup (failover) mail server.

https://www.howtoforge.com/postfix_backup_mx