It sounds like LISTSERV might not be able to resolve the address of your
mail server.  Please check the SMTPS#1-yymmdd.LOG files to see if there's
any indication of this (e.g., "Can't resolve MAIL.ECONOPHONE.COM - will
try again in one minute").  There may be a DNS problem that isn't evident
when you send mail via your mail client.  Also, if MAIL.ECONOPHONE.COM is
set to reject "relayed" mail, I can see how this might be happening.

Nathan