Okay, I give up. Apparently MCI has an SMTP filter either on the domain netins.net or on something in the 167.142.0.0/16 network on postoffice.reston.mci.net. [167.142.225.4]$ 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 11:2) HELO netins.net 250 postoffice.Reston.mci.net Hello kevin@worf.netins.net [167.142.225.4], pleased.. MAIL FROM:<kevin@netins.net> 550 Access denied QUIT I've called the iNOC, they sent me to a 'systems group'. The systems group listened carefully and wrote down the error code '550 Access denied' before shuffling me off to the Reston office. There, I spoke to a very nice sales person, who searched a list of voicemail boxes she might forward me to. We pay MCI good money, and find ourselves unable to send mail to trouble@mci.net or other MCI addresses. Now I'm stuck in the MCI phone maze. If there was an incedent warrenting an SMTP filter, we'd certainly deal with it if notified. I know if that happened with one of our downstream customers, we'd actually contact them before throwing in filters. Anyone know who takes care of postoffice.reston.mci.net ? Kevin