We use Sendmail as our backup server
There is a thread on configuring it to check addresses against zimbra.
If you set up local LDAP and sync against Zimbra LDAP you could automatically keep a local copy of the mail user data base. After that you are on to the Sendmail HOW-TOs to get anti-spam etc.
Sendmail is not as memory hungry as Zimbra as it is purely an MTA
Alternatively you could configure a Community Edition and replicate the Postfix config on a standard Linux install (against a local replicated LDAP) |