Roundcube mail server using centOS or Debian OS and Piller server implementation

We have a Debian open source mail server. For backup purpose, we have to implement pillar server and mail have to same in compress format in pillar to get more space. Whatever mails sending and landing to Debian server, one copy have to reach pillar server. Moreover, latest 3 months have to store in Debian server (primary server) and 3 months old mails have to store in Pillar server (backup server) and whenever user wants to check 3 months old mail then they should have a provision to login into pillar server to check their old mails.