Hello Dmitry

Perhaps you could figure out what is happening by using Rick Strahl's West wind internet and client tools.

>Hi,
>
>How do you authenticate an email address for MS 365 email server?
>
>Here is an example.
>In my ASP.NET application, on my computer, I can set to use the SMTP "Smtp.office365.com", port 587, enable SSL and TSL12 and my emails go though, no problems.
>I have a couple of customers who also set the SMTP to the same "Smtp.office365.com", etc. And emails go through, no problems.
>
>But I have one customer who get an error 535 5.7.3 Authentication unsuccessful. I told them many time that this means that the Sender email is not authenticated against (if this is the right term) "Smtp.office365.com". But they do not seem to know how to get around this.
>So, my question is, what steps they need to take to authenticate the sender email against the Office365 server? How do you do it?
>TIA