so outlook has you flagged as a spammer. Perhaps have a chat to them. When I googled the error this

so outlook has you flagged as a spammer. Perhaps have a chat to them.

When I googled the error this perhaps useful information came up in the backup4all web site https://www.backup4all.com/how-to-configure-and-use-office365-smtp-server-in-backup4all-kb.html

n this case, you need to first confirm your office365 email account. To do that, just login to https://outlook.live.com with your email and password. A phone number for confirmation will be required. After the account is confirmed, you can try again to send a notification test email from Backup4all.

I suggest you try and confirm your account as described.