Kevin.. it works if you give the proper machine name in the HELO
at least for me:
telnet postoffice.reston.mci.net. 25
Trying 204.70.128.20...
Connected to postoffice.reston.mci.net.
Escape character is '^]'.
220 postoffice.Reston.mci.net ESMTP Sendmail 8.8.5/8.8.5; Wed, 24 Sep 1997 14:44:12 -0400 (EDT)
HELO puck.nether.net
250 postoffice.Reston.mci.net Hello jared@puck.nether.net [204.42.254.5], pleased to meet you
MAIL FROM:<jared@cic.net>
250 <jared@cic.net>... Sender ok
quit
221 postoffice.Reston.mci.net closing connection
Connection closed by foreign host.
Kevin Houle boldly claimed:
MCI's iNOC pretty much confirmed there was a filter in place,
as did the systems person I finally reached thanks to some
key responses from this list. Probably a spam relay block of
some kind. The '550 Access denied' reponse is that they use
in their SMTP filters. postoffice.reston.mci.net was down for
two hours and counting when I got them on the phone, so with
local MCI users not being able to send mail, I was not a high
priority 
At any rate, the 'HELO netins.net' statement is a result of
host masquerading (DMnetins.net). Same thing happens with a
FQDN :
220 postoffice.Reston.mci.net ESMTP Sendmail 8.8.5/8.8.5; Wed, 24 Sep 1997 15:16
:25 -0400 (EDT)
HELO worf.netins.net
250 postoffice.Reston.mci.net Hello kevin@worf.netins.net [167.142.225.4], pleas
ed to meet you
MAIL FROM:<kevin@netins.net>
550 Access denied
Kevin
FWIW I too see this when attempting from one address, but not from
another. Both addresses are in MCI customer-owned ip blocks. Both
addresses have correct double-reverse lookups. It's more than just a bit
annoying ... especially when I receive mail from MCI's noc and cannot
reply to it unless I go through hoops to route it through another box.
Dean