While reviewing security policies, a company's IT administrator realizes that they need to improve the way they protect confidential emails being sent outside the organization. What method should the administrator recommend to ensure both the confidentiality and integrity of the emails?
Enforce a policy of two-factor authentication for email account access.
Restrict email communications to internal servers only, barring all external communication.
Schedule regular data backups of all email correspondence.
Implement email encryption and require digital signatures.