zimbra is a nice email server available as open source software.
here is a tip to copy mails from one user account to another user account.
run the following commands in the server.
su – zimbra
zmmailbox -z -m firstname.lastname@example.org getRestURL “//?fmt=tgz” > /tmp/v.tar.gz
zmmailbox -z -m email@example.com postRestURL “//?fmt=tgz&resolve=reset” /tmp/v.tar.gz
these commands copy all the emails from the olduser account to newuser account.