sync mail password

Hello,
I want to setup two seperate ispconfig e-mail hosting server with a haproxy server for a failover system.Everything is working fine but;
What happen If a user changes his mail password before crash?
Haproxy wil redirect all smtp/pop traffic to second server alive.Email client will not work because of wrong password.It is old password actually.
I need to find a way about synchronize mail passwords on all ispconfig server.
What is the best way to setup like this system?
Regards,

 

thank you.