Kolab 2.1: kolabmailboxfilter error

[Edward Kehler]

I'm in the process of migrating from a Kolab 2 setup (non-OpenPkg on
Mandriva) to Kolab 2.1 (OpenPkg on Debian) and am getting the following
for incoming mail in 2.1:
-----
Jun 07 16:41:22 mailhost <info> postfix/pipe[4808]: 08B14A7060A:
to=<xxx at yyy.com>, relay=kolabmailboxfilter, delay=0, status=deferred
(temporary failure. Command output: Failed to set recipient: 4.3.0 System
I/O error, code 451 )
-----
Any suggestions as to what "4.3.0 System I/O error, code 451" indicates?

Thanks!
Edward Kehler