E-mail bounces, but address does exist

I am getting the following error when I send a mail from one of my Linux
servers, using an exchange server inside our firewall to relay the
message - the mail is sent to an internal address as well:

May 15 12:11:14 oraapp1 postfix/pickup[2697]: 65C4F829: uid=0
from=<root>
May 15 12:11:14 oraapp1 postfix/cleanup[32552]: 65C4F829:
message-id=<20080515101114.GA32548 <at> oraapp1.caretech.co.za>
May 15 12:11:14 oraapp1 postfix/qmgr[5165]: 65C4F829:
from=<root <at> oraapp1.caretech.co.za>, size=733, nrcpt=1 (queue active)
May 15 12:11:14 oraapp1 postfix/local[32557]: 65C4F829:
to=<membership <at> caretech.co.za>, relay=local, delay=0, status=bounced
(unknown user: "membership")
May 15 12:11:14 oraapp1 postfix/cleanup[32552]: 6D1DDB5C:
message-id=<20080515101114.6D1DDB5C <at> oraapp1.caretech.co.za>
May 15 12:11:14 oraapp1 postfix/qmgr[5165]: 6D1DDB5C: from=<>,
size=2452, nrcpt=1 (queue active)
May 15 12:11:14 oraapp1 postfix/qmgr[5165]: 65C4F829: removed
May 15 12:11:14 oraapp1 postfix/local[32557]: 6D1DDB5C:
to=<root <at> oraapp1.caretech.co.za>, relay=local, delay=0, status=sent
(delivered to mailbox)
May 15 12:11:14 oraapp1 postfix/qmgr[5165]: 6D1DDB5C: removed

The address membership <at> caretech.co.za does exist.  I can send mail from
Outlook to this address, but not from the Linux server.

When I specify my Outlook address as the reply-address, I get the
following mail back:

From: System Administrator