5.1.0 Sender denied

Hi -
I have a client running Exchange 2010 under SBS 2011.  There are two senders from one firm with the same domain and very similar email addresses.  From one sender mail can be sent to the client but from the other we are getting the following message -

could not be delivered. The problem appears to be :
-- Recipient email server rejected the message

Additional information follows :
-- 5.1.0 Sender denied

This condition occurred after 0 attempt(s) to deliver over a period of 0 hour(s).

I lower the reputation threshold but was wondering if anyone has an idea why this is happening.  Two users from the same firm - one is able to send mail and the other one has the mail rejected.

Who is Participating?
David Johnson, CD, MVPOwnerCommented:
the end user may have blocked the email address of the one that is being refused.

I'm guessing this is only to one domain that you are getting this error
webentprAuthor Commented:
Thank you so much - This is exactly the solution.  Nicely Done - Thanks
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