One of the things I prefer to set up on all Multifunction Printers is Scan-to-email. The config is much easier to do and you can add and remove users quickly as required. The problem is when all Scan-to-email functionality just stops working for no reason at all, troubleshooting can become tricky. This was the situation when Microsoft disabled all SMTP authentication within the organization.
Send-MailMessage -Body "Test" -From "firstname.lastname@example.org" -To "email@example.com" -Credential (Get-Credential) -SmtpServer "smtp.office365.com" -UseSsl $true -Port 587
This gives you an indication of what the actual problem is:
The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.57 Client not authenticated to send mail. Error: 535 5.7.139 Authentication unsuccessful, the request did not meet the criteria to be authenticated successfully. Contact your administrator.
To enable SMTP Authentication, browse to the user account on the Microsoft Admin Portal.
Choose Mail and Manage Email apps then Select Authenticated SMTP: