I have postfix 2.3.3 installed and configured on a home machine
(running debian etch, if it matters). Since I have a dynamic IP, I'm
using the sender-dependent relay host and authentication features to
relay my mail out through one of two servers on which I have accounts.
Of the two, one works fine, while the other consistently gives me
"relay access denied" bounce messages, which I assume means I'm not
authenticating properly with it. How can I find out if the smtp client
is attempting to authenticate and, if so, why it isn't succeeding? All
the logs and reports I've looked at merely have the failure message
itself, not the previous history. I've tried using sendmail -v and
debug_peer_list, perhaps incorrectly.

Example error message:

Sep 21 20:07:06 spip postfix/smtp[20508]: C23893A626: to=<XXX@mit.edu>,
relay=in1.smtp.messagingengine.com[66.111.4.73]:25, delay=0.89,
delays=0.03/0.03/0.32/0.51, dsn=5.0.0, status=bounced (host
in1.smtp.messagingengine.com[66.111.4.7
3] said: 554 <XXX@mit.edu>: Relay access denied (in reply to RCPT TO
command))

To add insult to injury, it was working last week, and I can't figure
out what relevant change I might have made.

Any guidance would be much appreciated.

Ian