copying mailboxes

[Matt McCutchen]

Rsync can handle directories.  However, the only step of the
communication that is incremental is the part between the sending
rsync process and the receiving rsync process.  In your setup, the
receiving rsync will have to rewrite all changed files in their
entirety using the non-incremental NFS protocol.  To get any benefit
from rsync's incremental transfer, you need to have the receiving
rsync run on the new mail server using either SSH or an rsync daemon.
SSH is probably easier.  The command to issue on the sending machine
looks like this:
    rsync <options> /var/spool/mail/ root at new.mail.server.com:/var/spool/mail/

Matt