Just to clarify, there is nothing wrong with the way QF is sending emails via SMTP. We've revisited this numerous times, the problem is different mail providers have subtle differences in the way they authenticate incoming requests. We have configured our SMTP interface as per the recommended .NET specification.
The error message you refer to above indicates that Outlook have blocked our incoming requests. However I do see some QF users with verified credentials against smtp-mail.outlook.com. So I suspect the host name and port number is correct but Outlook are blocking the request as they don't recognise our IP.