I have been reading documentation after documentations on the forum. I found several possible solutions and none worked. I have installed bind and configured DNS on the same box. Made so many changes I lost track. My problem still persist.

Dec 18 16:06:59 zimbra postfix/smtp[29307]: 5C40F1C8B4F3: to=, relay=[], delay=21, status=deferred (host[] said: 451 4.5.0 Error in processing, id=28175-07, parts_decode_ext FAILED: Unix utility file(1) not available, but is needed at (eval 48) line 109. (in reply to end of DATA command))

I cannot send or receive mail even within the same domain.
Any help will be greatly appreciated.