I have several domains lets say domain1.com and domain2.com these domains have seperate static ips, physical locations and even different isps. I have zimbra setup on one box already and am in the process of setting up the other box i wish to use these machines

1. as backup mx servers for each other
2. as completely redundant servers ie each user will be on both machines if one fails the other takes over. Is this possiable using zimbra or is there a better way to achive this?

Please forgive me if this is covered in the docs or on the lists somewhere if so just point me in the right direction.