Postifx as SMTP AUTH client

[J. K. Cliburn]

I'm pretty new to configuring mail servers, and I'm running into
issues trying to get outbound mail to work.  I've configured Postfix
to (I think) request smtp auth at my ISP's mail server in an attempt
to get him to relay my mail, but I'm not sure the authentication
attempt is happening.

A few background items...
1) PPPoE DSL connection, dynamic IP.
2) DynDNS/ddclient correctly working to keep domain name and IP
address in sync.
3) Postfix on home LAN server correctly configured to send and receive
internal mail.

I send a message from root at hogchain.net to jcliburn at gmail.com...  The
ISP's mail server unceremoniously bounces it thusly.

Apr 10 12:17:34 osprey postfix/pickup[4586]: B48F46CE549: uid=0 from=<root>
Apr 10 12:17:34 osprey postfix/cleanup[4622]: B48F46CE549:
message-id=<20060410171734.B48F46CE549 at osprey.hogchain.net>
Apr 10 12:17:34 osprey postfix/qmgr[4587]: B48F46CE549:
from=<root at hogchain.net>, size=313, nrcpt=1 (queue active)
Apr 10 12:17:40 osprey postfix/smtp[4624]: B48F46CE549:
to=<jcliburn at gmail.com>, relay=mx00.mail.bellsouth.net[205.152.58.32],
delay=6, status=bounced (host mx00.mail.bellsouth.net[205.152.58.32]
said: 550 relaying mail to gmail.com is not allowed (in reply to RCPT
TO command))
Apr 10 12:17:40 osprey postfix/cleanup[4622]: 5E98B6CE563:
message-id=<20060410171740.5E98B6CE563 at osprey.hogchain.net>
Apr 10 12:17:40 osprey postfix/qmgr[4587]: B48F46CE549: removed
Apr 10 12:17:40 osprey postfix/qmgr[4587]: 5E98B6CE563: from=<>,
size=2187, nrcpt=1 (queue active)
Apr 10 12:17:40 osprey postfix/smtp[4624]: 5E98B6CE563:
to=<root at hogchain.net>, relay=mx01.mail.bellsouth.net[205.152.58.33],
delay=0, status=bounced (host mx01.mail.bellsouth.net[205.152.58.33]
said: 550 relaying mail to hogchain.net is not allowed (in reply to
RCPT TO command))
Apr 10 12:17:40 osprey postfix/qmgr[4587]: 5E98B6CE563: removed

I know the server is saying "relaying mail to [wherever] is not
allowed," but does that mean not allowed at all, or not allowed
without smtp authentication?

So, hoping it's the latter, I setup smtp auth on my postfix, reasoning
that if the ISP requires authentication whenever I fetch my personal
email from him (using a client like thunderbird), then maybe he
requires an authentication before he'll relay.  But it doesn't help. 
I still get bounced.

Interestingly, here's a telnet session with the ISP's mailserver:

[root at osprey ~]# telnet 205.152.58.33 25
Trying 205.152.58.33...
Connected to 205.152.58.33.
Escape character is '^]'.
220 ibm30aec.bellsouth.net ESMTP server ready Mon, 10 Apr 2006 13:28:30 -0400
EHLO bellsouth.net
250-ibm30aec.bellsouth.net
250-HELP
250-PIPELINING
250-DSN
250-8BITMIME
250 SIZE 26214400
AUTH PLAIN <<<base64-encoded-username/password>>>
502 Command is locally disabled
quit
221 ibm30aec.bellsouth.net ESMTP server closing connection
Connection closed by foreign host.

What does the response "Command is locally disabled" mean?  I'm
confused by it, because the ISP requires that I authenticate when I
check my personal mail.  How could the AUTH command be disabled at his
end in this case?

I'm missing something really fundamental.  Any hints or pointers appreciated.

Thanks,
Jay