In addition to Hank's suggestions, they may need to check that SMTP is enabled: https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/authenticated-client-smtp-submission

>Hi Hank,
>
>Thank you for your message. I will pass this to the customer.
>
>>The first step I would take is send an email to that address to make sure it doesn't bounce.
>>
>>The second step is to verify the account in the Microsoft 365 Admin Center. And then change the password to what they think it already is.
>>
>>If all that is good or fixed and there is still an error, what I would do is use Chilkat SMTP activex control (https://www.example-code.com/foxpro/smtp_office365.asp) with the mailman object VerboseLogging property set to 1.
>>
>>This will tell you exactly where the problem happened.
>>
>>Hank
>>
>>>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