Mail from server can't be authenticated (550 error) [SOLVED]

What's up, folks. I've been banging my head against the wall for awhile with this one. Ok, so I have an email configuration using Postfix and Courier, which seems to be up and running ok. Messages are being sent, but some servers are bouncing them back to me saying the user does not exist. For example

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

 jedi@ricketycontraption.com
   No Such User Here

------ This is a copy of the message, including all the headers. ------

Return-path: <usr@domain.com>Received: from [24.42.78.153] (helo=[10.0.1.2])
    by fst02.justhost.com with esmtp (Exim 4.69)
    (envelope-from <usr@domain.com>)
    id 1OFtsZ-00056J-MV
    for jedi@ricketycontraption.com; Sat, 22 May 2010 13:56:59 -0500
From: Ro <usr@domain.com>Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Subject: testing
Date: Sat, 22 May 2010 14:56:59 -0400
Message-Id: <f9d2f64a-c67d-4b1e-ae4c-12ff7ad12eef@domain.com>To: jedi@ricketycontraption.com</f9d2f64a-c67d-4b1e-ae4c-12ff7ad12eef@domain.com></usr@domain.com></usr@domain.com></usr@domain.com> 

I've checked my mx records, which point to the correct place, updated my DNS, set my reverse DNS a couple of days ago to give it time to propogate, and still nothing. I'm not getting any mail errors or warnings and all of my mail domain urls are matching up. Here is an excerpt from my mail logs which.

May 22 15:11:34 durendal postfix/qmgr[2774]: 57631A697: from=, size=1307, nrcpt=1 (queue active)
May 22 15:11:34 durendal amavis[2216]: (02216-01) Passed CLEAN, LOCAL [127.0.0.1] [24.42.78.153] -> , Message-ID: <2c10138d88150a94db3e88b4e0a27eae.squirrel@ricketycontraption.com>, mail_id: 4lVSxyNpDdgq, Hits: -0.999, size: 792, queued_as: 57631A697, 436 ms
May 22 15:11:34 durendal postfix/smtp[2894]: B1DD2A696: to=, relay=127.0.0.1[127.0.0.1]:10024, delay=0.71, delays=0.23/0.04/0.01/0.43, dsn=2.0.0, status=sent (250 2.0.0 Ok, id=02216-01, from MTA([127.0.0.1]:10025): 250 2.0.0 Ok: queued as 57631A697)
May 22 15:11:34 durendal postfix/qmgr[2774]: B1DD2A696: removed
May 22 15:11:34 durendal imapd: LOGOUT, user=jedi@ricketycontraption.com, ip=[::ffff:127.0.0.1], headers=0, body=0, rcvd=660, sent=203, time=0
May 22 15:11:34 durendal imapd: Connection, ip=[::ffff:127.0.0.1]
May 22 15:11:34 durendal imapd: LOGIN, user=jedi@ricketycontraption.com, ip=[::ffff:127.0.0.1], port=[45997], protocol=IMAP
May 22 15:11:34 durendal imapd: LOGOUT, user=jedi@ricketycontraption.com, ip=[::ffff:127.0.0.1], headers=1710, body=0, rcvd=296, sent=4010, time=0
May 22 15:11:34 durendal postfix/smtp[2904]: 57631A697: to=, relay=playvicious.com[69.175.25.18]:25, delay=0.35, delays=0.03/0.01/0.26/0.05, dsn=5.0.0, status=bounced (host playvicious.com[69.175.25.18] said: 550-Verification failed for 550-No Such User Here 550 Sender verify failed (in reply to RCPT TO command))
May 22 15:11:34 durendal postfix/cleanup[2892]: B2A8AA698: message-id=<20100522191134.B2A8AA698@durendal.ricketycontraption.com>
May 22 15:11:34 durendal postfix/bounce[2907]: 57631A697: sender non-delivery notification: B2A8AA698
May 22 15:11:34 durendal postfix/qmgr[2774]: B2A8AA698: from=<>, size=3592, nrcpt=1 (queue active)
May 22 15:11:34 durendal postfix/qmgr[2774]: 57631A697: removed
May 22 15:11:35 durendal postfix/pipe[2908]: B2A8AA698: to=, relay=maildrop, delay=0.34, delays=0.18/0.01/0/0.15, dsn=2.0.0, status=sent (delivered via maildrop service)
May 22 15:11:35 durendal postfix/qmgr[2774]: B2A8AA698: removed
May 22 15:11:36 durendal imapd: Connection, ip=[::ffff:127.0.0.1]
May 22 15:11:36 durendal imapd: LOGIN, user=jedi@ricketycontraption.com, ip=[::ffff:127.0.0.1], port=[45998], protocol=IMAP
May 22 15:11:36 durendal imapd: LOGOUT, user=jedi@ricketycontraption.com, ip=[::ffff:127.0.0.1], headers=2026, body=0, rcvd=296, sent=4545, time=0
May 22 15:11:38 durendal imapd: Connection, ip=[::ffff:127.0.0.1]
May 22 15:11:38 durendal imapd: LOGIN, user=jedi@ricketycontraption.com, ip=[::ffff:127.0.0.1], port=[45999], protocol=IMAP
May 22 15:11:38 durendal imapd: LOGOUT, user=jedi@ricketycontraption.com, ip=[::ffff:127.0.0.1], headers=0, body=2556, rcvd=178, sent=3996, time=0
May 22 15:13:42 durendal imapd: Connection, ip=[::ffff:127.0.0.1]
May 22 15:13:42 durendal imapd: LOGIN, user=jedi@ricketycontraption.com, ip=[::ffff:127.0.0.1], port=[46004], protocol=IMAP
May 22 15:13:42 durendal imapd: LOGOUT, user=jedi@ricketycontraption.com, ip=[::ffff:127.0.0.1], headers=0, body=0, rcvd=87, sent=391, time=0
May 22 15:15:02 durendal imapd: Connection, ip=[::ffff:127.0.0.1]
May 22 15:15:02 durendal imapd: Disconnected, ip=[::ffff:127.0.0.1], time=0

I'm so close to getting this to work as it seems to send to gmail fine, but only craps out on particular servers that require verification.