What is the standard method for securing individual e-mail messages sent between a company and other users that do not use that e-mail server?

A.
Invoke encryption at the e-mail server

B.
Invoke encryption on each client

C.
Filter firewall port 42 on the company firewall

D.
Store all e-mail messages on a separate partition

Explanation:
E-mail messages can be secured be encrypting them. A client application at receiver of the e-mail could then decrypt the e-mail messages. A possible solution would be to use PGP (Pretty Good Privacy) at both the server and the client.
Incorrect Answers:
A: Encryption would have to be enabled both the clients and at the server.
C: Instead of using the standard e-mail port, port 25, a secure port, port 42 for example, can be used to enhance security of e-mail. In this scenario, however, this is not applicable since some users don’t use this e-mail server.
D: Locally securing the e-mail messages wouldn’t help much when they are transferred.