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
<email@example.com>: host outlook.office365.com[188.8.131.52] said: 530
Client was not authenticated (in reply to MAIL FROM command)
Is this our issue or the receiving end? anything I can try?