According to the CRISC Review Manual (Digital Version), the most effective approach to mitigate the risk associated with data loss due to users sending sensitive information by email without using encryption is to block unencrypted outgoing emails which contain sensitive data. This is an example of a risk avoidance strategy, which aims to eliminate the risk by removing the source of the risk or the activity that causes the risk. Blocking unencrypted outgoing emails which contain sensitive data can prevent unauthorized access, disclosure, modification or destruction of the sensitive information, and thus protect the confidentiality, integrity and availability of the data. This approach can also deter users from violating the encryption policy and enforce compliance with the security standards and regulations.
References = CRISC Review Manual (Digital Version), Chapter 3: IT Risk Response, Section 3.3: Risk Response Options, pp. 167-1681