Office365 mail is on smtp.office365.com port 587 and uses STARTTLS
What you are using (I assume xxx is your domain name) is the unauthenticated mail server for use with aging systems that cannot authenticate to industry standards, which is limited and cannot, at least by default, do SMTP relay (Email to external domains)
The exact thing happens when you use gmails ASPMX server.
If you don't want to have the printer use the real server and a real email account (Because it will need to authenticate to it) then you need to setup a Connector on your Office365.