I have this error aswell and I've tried everything suggested here. I'm using Ubuntu.
Code:
Starting mta...Done.
zimbra@zimbra:~$ postqueue -p
-Queue ID- --Size-- ----Arrival Time---- -Sender/Recipient-------
BB3D454062 1508 Wed Mar 14 19:24:37 nilo@domain.se
(host 127.0.0.1[127.0.0.1] said: 451 4.5.0 Error in processing, id=07961-06, parts_decode_ext FAILED: Unix utility file(1) not available, but is needed at (eval 51) line 111. (in reply to end of DATA command))
admin@zimbra.domain2.se
DE48254061 1341 Wed Mar 14 19:24:37 minaola@hotmail.com
(host 127.0.0.1[127.0.0.1] said: 451 4.5.0 Error in processing, id=07959-05, parts_decode_ext FAILED: Unix utility file(1) not available, but is needed at (eval 51) line 111. (in reply to end of DATA command))
admin@zimbra.domain2.se
-- 3 Kbytes in 2 Requests. I've edited the mail accounts they all work