Blocking SMTP from personal MS accounts in Outlook

Products

• Data Loss Prevention Endpoint Prevent
• Data Loss Prevention

Issue/Introduction

You want to block all SMTP from endpoint devices unless the sender's email is trusted.
For example, a user adds a personal MS account mailbox to the Outlook client on their corporate laptop and then sends classified information to an external email recipient. 
This should be blocked.
However, the user should still be able to send emails from his/her corporate account.

Resolution

1. Create a policy to block the "Protocol is Email/SMTP".
2. Add a rule for "Protocol > Protocol or Endpoint Monitoring > select Email/SMTP".
3. Add a Block response rule.
4. On the Groups tab, add an exception for "Sender/User Matches Pattern".
5. Either select an already configured "Reusable Sender Pattern" or add your "Sender Pattern" with the "<company>.<domain>", etc., partial email addresses.