Plesk and failover between two systems (same IP configured on two servers)

[Thomas Becker]

Hello,
I have a simple problem but I do not know how to fix it.
I have two systems with Plesk and Postfix on CentOS 6. On both systems there is the same domain and IP configured:

System A:
Customer with Domain customer.com and IP 11.22.33.44

System B (failover):
Customer with Domain customer.com and IP 11.22.33.44
Mail service and DNS switched off for customer.com in Plesk

This is because of the case of a server failure. In such case we can simply switch the routing of the IP 11.22.33.44 from System A to System B and the website customer.com is available again.

The problem is that Postfix on System B seems to reject mails from System A to any domain on System B. In mail log on System A there is something like "service not available". This is default behavior of Plesks Postfix integration and the anti-spam mechanisms if a system is not authorized to send mails.
When I remove the IP 11.22.33.44 from System B in Plesk (Tools & Settings --> IP Addresses), the mails are delivered.

Plesk seems to write the IP 11.22.33.44 in some config files but which one?
If I know the behavior I can figure out how to solve this.
Any idea how to proceed?

Many Thanks.