Securing SMTP traffic has been a concern for many years. Nowadays, many servers support opportunistic Transport Layer Security (TLS) where the sending server first attempts to secure the path that emails take when they travel to recipient email systems by using encryption. However, this is not always possible and emails end up being sent in clear text.
As you will see in the last recipe of this chapter, S/MIME can be used to digitally sign and encrypt emails, but if certificates from an internal PKI are used, external recipients will likely not trust them. Additionally, implementing S/MIME on an enterprise scale is not always easy.
Domain security provides a low-cost alternative to S/MIME and other message-level security solutions, by helping secure SMTP traffic between two Exchange organizations. Its advantage is that it is configured on a server level...