Depending on the industry and the countries you are doing business applicable laws, regulations and frameworks of best practice can demand e-mail compliance in the form of
• User authentication in order to create and send emails
• Receiver identification, for instance trough corporate user authentication for reading emails
• Email signing (non-repudiation), for instance through Public Key Cryptography as found in PGP
• Electronic message encryption, also found in PGP Desktop and similar products
• Legal disclaimers in the email appendix
• Message filtering for outbound emails (sensitive data, company secrets)
• Automated secure email archiving preventing alterations and malware damage
• Fast retrieval of archived electronic messages
• Secure deletion of emails after the retention period
• Email logging and reporting
It is important that your organizations technical means of email compliance are scalable and ready to adapt to new regulations.