I have an Exchange 2010 server setup and I installed a ShoreTel phone system yesterday. When it comes to the SMTP feature, I have receive connectors that have been working and we are not an open relay.
For example, the shoretel server smart host set to smtp.office365.com with basic authentication and TLS. If I send it to an email address associated with the O365 account it works to various email accounts except my domain.
Something must be blocking it. I see that the eml message is stuck in the "queue" folder under IIS.