I have been battling a 550 Relaying is Prohibited problem for a while now
with our Listserv Lite installation.  Here's what I have done so far.  I
have listserv set to go to our Exchange 5.5 box for SMTP Forward.
Everything works great as long as you are emailing on our domain internally.
The problem is with outside emails...in the SMTPS#1-XXX.log is states the
550 error.  I understand that I must set up routing on the Exchange server's
Internet Mail Service Routing tab.  We keep the first checkbox "Hosts and
clients that successfully authenticate" box checked all the time, even
before the listserv installation.  This is for security reasons.  I have
tried before and now unsuccessfully to check the second box "Hosts and
clients with these IP addresses" and entered the internal IP of the server
running listserv.  I have also entered the external IP address of that
server as well (the IP I would get if I were to go home and ping the server
-- it is one of our webservers).  Neither have worked.

About two months ago I tried checking the third box "Hosts and clients
connecting to these internal addresses" and putting the exchange server
address (internal) in it and everything worked fine for a couple of weeks.
Then we started having problems with us being listed as a relay server and
we could not send emails to certain domains from our Exchange server.
Needless to say I have removed that setting.

The only thing I have found to do is to enable the "Hosts and clients with
these IP addresses" and enter the IP of the server running listserv.  I
haven't found any other suggestions.  If anyone has any suggestions or ideas
I would greatly appreciate them.  I have already posted several questions to
the Listserv-Lite list and have gotten plenty of answers, but none that seem
to work for me.  This list was suggested by someone on the Listserv-Lite
list.  Thanks in advance for any help anyone has to offer.  I appreciate it!
Thanks!