The only thing that I can conjecture is that there is a problem
"understanding" CNAME records by one of your MTAs

Query:Mail Exchange (MX):LISTS.FBCLA.ORG
LISTS.FBCLA.ORG CNAME   tahoe.FBCLA.ORG
FBCLA.ORG       SOA     udns1.ultradns.net      john.benza.FBCLA.ORG
Complete: LISTS.FBCLA.ORG

Query:Address (A):tahoe.FBCLA.ORG
Authoritative Answer
tahoe.fbcla.org A       66.7.238.246
fbcla.org       NS      udns2.ultradns.net
fbcla.org       NS      udns1.ultradns.net
udns2.ultradns.net      A       204.74.101.1
udns1.ultradns.net      A       204.69.234.1

At 17:36 07/20/2003 Sunday, Roberto Almanza wrote:
 >Hi Pete,
 >
 >I am trying to mail to LISTSERV and post to my test list using my
 >pacbell-yahoo email address but I am always getting the "550 Recipient not
 >local" error message. No problem for me when using my Hotmail account as
 >well as for other users using email addresses from other ISPs. Not working
 >for AOL accounts and a few other ISPs. I susbscribed both my pacbell-yahoo
 >and Hotmail accounts thru the listserv web interface, and I am
 >successfully receiving postings from users that have no problem. But I
 >can't post to the list nor send commands to listserv using my
 >pacbell-yahoo account.