<div>
Thanks, I have not contacted their support, I will need to try and find who supports them.
</div>


Thanks, I have not contacted their support, I will need to try and find who supports them.

<div>
<div class="content wysiwyg-content">
We are sending an email to multiple users all of which go thru fine except for one, we get the following error: &nbsp;<br />
<br />
This is the mail system at host zixvpm.wmmhc.loc.<br />
<br />
I'm sorry to have to inform you that your message could not be delivered<br />
to one or more recipients. It's attached below.<br />
<br />
For further assistance, please send mail to postmaster.<br />
<br />
If you do so, please include this problem report. You can delete your<br />
own text from the attached returned message.<br />
<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;The mail system<br />
<br />
&lt;smalloy@wmmhc.org&gt;: host outlook.office365.com[132.<wbr />245.21.82]<wbr /> said: 530<br />
5.7.1<br />
&nbsp; &nbsp; Client was not authenticated (in reply to MAIL FROM command)<br />
<br />
<br />
Is this our issue or the receiving end? &nbsp;anything I can try?
</div>
</div>


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

<smalloy@wmmhc.org>: host outlook.office365.com[132.245.21.82] said: 530
5.7.1
    Client was not authenticated (in reply to MAIL FROM command)


Is this our issue or the receiving end?  anything I can try?

Sending Email one address keeps getting undelivered mail returned to sender

An email client, email reader or more formally mail user agent (MUA) is a computer program used to access and manage a user's email. A web application that provides message management, composition, and reception functions is sometimes also considered an email client, but more commonly referred to as webmail.

Internet / Email Software

Windows Server 2003 was based on Windows XP and was released in four editions: Web, Standard, Enterprise and Datacenter. It also had derivative versions for clusters, storage and Microsoft’s Small Business Server. Important upgrades included integrating Internet Information Services (IIS), improvements to Active Directory (AD) and Group Policy (GP), and the migration to Automated System Recovery (ASR).