Exchange Server 2010 Restrictions

I manage an exchange server 2010 The company that handles our faxes forwards them to us via email.  Recently they told me we have a restriction set on our server and couldn't forward them to us.  So I checked The exchange management console.  There are no:

1) Hub transport rules
2)mailbox restrictions for the specific user account
3)quota limits or message size limit restrictions

The only other place I thought to check was the exchange management shell.  But I'm not seeing in there where any restrictions or blocked users/domains are that pertain to the fax company.  What command should I be typing in the shell to check for sender restrictions.  And, have I checked all the correct locations for sender or mailbox restrictions.
Below is the telnet from their mail server to ours.

[11:49:16][labare@ops4.phx1 ~]$ telnet 25
Connected to
Escape character is '^]'.
220 Microsoft ESMTP MAIL Service ready at Tue, 1 Oct 2013 12:50:24 -0600
ehlo Hello []
250-SIZE 20971520
250 2.1.0 Sender OK
250 2.1.5 Recipient OK
354 Start mail input; end with <CRLF>.<CRLF>
This is a test message from xxxxxxx.
554 5.1.0 Sender denied
500 5.3.3 Unrecognized command

they used to send to the mailbox all the time now they are saying a restriction is on our side.  They can send to other boxes on our server.  And, The mail server sends and receives  emails from all other approved domains and senders
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David Paris VicenteSystems and Comunications  Administrator Commented:
Did you also check your receiver connectors.

In this case you are receiving and not sending?
Check the permissions on the default receiver and in the in the other receivers if yu have more than one.

And let us know.

"554 5.1.0 Sender denied" Can also indicate that the user used Outlook to put it in the Blocked Senders list. (see
Can you go to the user (who should be receiving the fax), and go through his/her Outlook to check it?

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Techguy13Author Commented:
Thank you both for your quick reply's I will check receiver connectors and I will be onsite on Monday to check user outlook settings.  I will update my findings.
Techguy13Author Commented:
Thank you both for your quick responses.  The end user accidentally blocked the email address.
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