Office 365 – 5.7.3 Authentication unsuccessful

Hi there,

We are getting the following error when trying to send email through O365.

5.7.3 Authentication unsuccessful [SY6PR01CA0010.ausprd01.prod.outlook.com]

Here is our current set up: (private details are obscured with –)

Outgoing Mail Server Hostname smtp.office365.com
Outgoing Mail Server Port 587
Envelope-From Email Address m—@——.co
Security STARTTLS
Authentication Login
Username m—@——.co
Password ?????????

We have the following TXT record in the DNS:

v=spf1 ip4:4-.2–.1–.47 include:spf.protection.outlook.com -all

We have included the above IP address (our web server IP address) in the O365 Connection filter as an allowed IP address to send from, and checked ‘enable safe list’.

In O365 we have enabled ‘SMTP auth’ for the mailbox.

We have checked the password many times and it is correct.

And yet the error persists.

Can you advise of anything we can try that can fix our issue?

I can provide a session transcript if required.

Thank you.