[Xymon] acknowledge alerts with email (postfix)

Hi

I've installed Xymon 4.3.3 on Centos 5.6 with postfix. Everything is working fine but I can't acknowledge alerts with email. I see the email is coming to the default location from postfix (/var/mail/xymon) and nothing happened...

Is there a howto or has anyone an idea how to configure this ?

Thanks Best regards

Frank