I have installed zimbra on our server,mails are not delivered and this error is logged to /opt/zimbra/log/mailbox.log.

May 9 23:19:00 mail postfix/smtp[14071]: 756FD209125: to=<xxx@x.x>, relay=[]:10024, delay=13121, delays=13121/0.01/0.01/0.05, dsn=4.5.0, status=deferred (host[] said: 451 4.5.0 Error in processing, id=03372-06, parts_decode_ext FAILED: Unix utility file(1) not available, but is needed at (eval 85) line 113. (in reply to end of DATA command))