I have found a solution to get Office365 working for UNRAID notifications.
Make sure you have an active outlook/exchange license on your account. I am using Exchange Online (Plan 1). Make sure to assign this to your notifications user. At this point, go in an incognito window and login to this user at login.microsoftonline.com. At this point, if you see a blue outlook icon on the side bar, click on it and setup your mailbox. If not, you will need to wait a few.
Anyway... first things first. I am running 6.10.0-rc4.
I am not sure if these settings will change in the future but here is what I have on the UNRAID side of things.
Preset Service: Office365
Sending Email Address: EMAIL@DOMAIN (Same as Username)
Email Recipients: MYEMAIL@DOMAIN
Priority in header: Yes
Email Subject Prefix: 'Unraid Status:'
Mail Server: SMTP-LEGACY.Office365.com
Mail Server Port: 587
Use SSL/TLS: Yes
Use STARTTLS: Yes
Define a TLS certificate: No
Authentication method: Login
Username: EMAIL@DOMAIN (Same as Sending email Address)
Password: MFA APP PASSWORD
**If you test this now, it will fail. I would open a new window and continue. We will eventually get back around to this.**
In admin.microsoft.com
Users -> Active users -> YOUR USER -> Mail -> Manage email apps -> Select Authenticated SMTP (This should work but I have all selected)
*To enable app passwords
Users -> Active users -> YOUR USER -> Scroll Down -> Manage multifactor authentication -> YOUR USER -> Enable -> enable multi-factor auth -> YOUR USER -> Enforce -> enforce mutli-factor auth
*To get an app password
Login to login.microsoftonline.com with your user -> click your profile -> view account -> Security Info -> Add method -> App password -> Add -> Give a name -> copy your app password and use this as the password in unraid SMTP settings.
In admin.exchange.microsoft.com
Settings -> Mail flow -> 'Turn on sending from aliases' + 'Turn on SMTP AUTH protocol for your organization' + 'Turn on use of legacy TLS clients'
Now for the very last thing to do. I cannot tell you how pissed I was when I figured this crap out. We need to turn of security defaults.
Head to azure.microsoft.com
sign in with your admin account and head into the azure portal. After that, we need to find the section to manage azure active directory. For me, this is in the middle of the screen with a white on blue button saying view.
From there... Properties, Manage Security Defaults, move the slider from Yes to No.
I am not sure why this prevents unraid from connecting but it does. Disable this at your own risk.
At this point, you should be able to click on test in unraid and after 2-5 seconds, it will give you a green "Sent mail from NOTIFICATIONS EMAIL".