Office365 has a few ways to configure SMTP clients. As Phil mentioned you need STARTTLS, but normally you DO need to authenticate. A thing to note however is that if you are authenticating as email@example.com then the FROM email address HAS TO BE firstname.lastname@example.org or Office365 will reject it. There is no send-as privileges in that configuration.
The alternative method (last I knew) was to configure a SMTP connector in Office365 which requires the sending network have a static IP address and that is used for authentication. Still needs STARTTLS but since you're coming from a known network you can send without plain auth. This setup will allow you to send as different FROM addresses.
This information is roughly a year old but I don't suspect much has changed recently --