Sending Email one address keeps getting undelivered mail returned to sender

We are sending an email to multiple users all of which go thru fine except for one, we get the following error:  

This is the mail system at host zixvpm.wmmhc.loc.

I'm sorry to have to inform you that your message could not be delivered
to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can delete your
own text from the attached returned message.

                   The mail system

<>: host[] said: 530
    Client was not authenticated (in reply to MAIL FROM command)

Is this our issue or the receiving end?  anything I can try?
Montana ManIT SupportAsked:
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Ben HartCommented:
I'd say it's on their end since it's their mailbox.  If they are running Exch2010 then they'll need to check their recieve connector.  Have you contacted their support/IT?

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Montana ManIT SupportAuthor Commented:
Thanks, I have not contacted their support, I will need to try and find who supports them.
Ben HartCommented:
Here's how you can resolve this if it happens on your end.
Taken from:

telnet 25
Which shows the following:
Then we say hi:
And because the laws of robotics tell it to do so, the server says hi back:
250 Hello
Then we try and relay to it:
And we get this error at some point in our smtp communication:
530 5.7.1 Client was not authenticated

Now, at this point we should look at the receive connector for Exchange, part of Hub Transport. To do so, open Exchange System Manager or Exchange Management Console and select Hub Transport under the Server Configuration section.

Here, locate your default receive connector (don’t do this on a send connector or you will create an open relay), right click it and then click on Properties.

At the Properties dialog, check the box for Anonymous users. This allows for another mail server to communicate with yours over smtp since that other server is basically just running through the message dialog we worked through earlier while telneted into the host. Click Apply to save the changes and for giggles go ahead and stop (disable) and start (enable) the connector.
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