One Account Unable To Email

Have them email you the bounce message, and post it here, obscuring identifiable information.  Make sure that the email address is actually correct in this bounce message.

Verify that this external user can send email to your other internal users (such as yourself when you receive this forwarded bounce message).

Are there any third party servers sitting between your email server and theirs, such as one that filters spam before it hits your site?

Here's a copy of the NDR:

Your message did not reach some or all of the intended recipients.
      Subject:  RE: Mom
      Sent:     4/4/2012 1:43 PM
The following recipient(s) cannot be reached:
      'John Smith' on 4/4/2012 1:43 PM
            550 5.2.1 <email@email.com>... User's mailbox is disabled



They get the same NDR if they email anyone within our domain.  We are using a hosted exchange service and they have verified that everything is fine on their end.

Your sender may be on a real time block list.  The message they are getting may just be generic, not really stating the true reason.

Ours is the only domain they cannot email to though. If they were on a block list wouldn't they have issues emailing other people also?

Try checking their domain here.  http://www.mxtoolbox.com/blacklists.aspx

Your email provider is probably doing the checking.  Not everyone they send to checks against these blacklists.

Sometimes hosted email providers make dumb mistakes and subscribe to blacklists that are unreliable, not updated properly, or have other problems.

You could contact your hosted provider to see if they are using rbl lists.

K. I checked on their domain using mxtoolbox as well as this tool http://domain-blacklist.e-dns.org/steeprock.ca
 and they are not blacklisted at all on any of those lists. I have a support ticket opened with our hosted exchange provider to confirm RBL, but I don't think the issue is there. I'm just trying to confirm whether it's our issue or theirs.

Thanks!

"however when one person tries to email us he receives a bounce back "

Its likely their Outlook AutoComplete Cache is incorrect, Broken, or corrupt.

Have them open a new email.
Then have them start typing in the email address
when the suggestino list come up, have them delete the suggested email address.

Close and reopen new email

have them manually type in the full, current email address.

Then send

Does the email go through?

This is happening from mulitple accounts behind that same domain. Anyone trying to email us from email@theiraccount.com get's that message so I don't think it's the autocomplete cache. I'm trying to track down the full headers from the message.

Here's the headers:


X-Spam-Flag: NO
X-POP-User: a_authtest.testsitesc0.hostopia.com
Return-Path: <MAILER-DAEMON>
Received: from mail141c0.megamailservers.com (localhost [127.0.0.1])
                by mail141c0.megamailservers.com (8.13.6/8.13.1) with ESMTP id q35JGAwY022339
                (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO)
                for <doug.bauman@steeprock.ca>; Thu, 5 Apr 2012 15:16:10 -0400
Received: from localhost (localhost)
                by mail141c0.megamailservers.com (8.13.6/8.12.2/Submit) id q35JG9MM022336;
                Thu, 5 Apr 2012 15:16:10 -0400
Date: Thu, 5 Apr 2012 15:16:10 -0400
From: Mail Delivery Subsystem <MAILER-DAEMON@mail141c0.megamailservers.com>
Message-Id: <201204051916.q35JG9MM022336@mail141c0.megamailservers.com>
To: email@email.com
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
                boundary="q35JG9MM022336.1333653370/mail141c0.megamailservers.com"
Subject: Returned mail: see transcript for details
Auto-Submitted: auto-generated (failure)
X-CSC: 0
X-CHA: v=1.1 cv=lh44UtWtnogc/zYhpl5FLdabZ4jnSR7RtjGfdbKRMqg= c=1 sm=1
                                a=nS2RiVxO7-MA:10 a=wPDyFdB5xvgA:10 a=97DucmoFUr-VG1hLoD4A:9
                                a=BDKbP5mgAAAA:8 a=9jqVpqLktJYWvzaML7MA:9 a=NHkSJBZ8GlqYRtDaUUcA:9
                                a=QEXdDO2ut3YA:10 a=WkljmVdYkabdwxfqvArNOQ==:117
X-CTCH-Spam: Unknown
X-CTCH-RefID: str=0001.0A020206.4F7DEF7A.0054,ss=1,re=0.000,fgs=256
X-WHL: SLR

The problem was caused and solved by the ISP. None of the solutions suggested worked.