to answer the OP, make sure you have /usr/sbin/sendmail and/or /usr/lib/sendmail set as symlink to /opt/zimbra/postfix/sbin/sendmail. Debian and hence ubuntu have a habit of insisting you keep an existing exim or postfix install (dpkg -l |grep [exim|postfix]) and thenthese conflict with zimbra postfix. you should find most other postfix binaries in /opt/zimbra/postfix which are safe to symlink although beware doing apt-get upgrades will overwrite these symlinks. someone else in the forum a while ago suggested a dummy mail package that was a great tip.
i would have thought zimbra postfix would resolve aliases through local maps first although i haven't looked so check yourself. in my experience, symlinking /usr/sbin/sendmail on various platforms usually fixes local deliveries.