Basic Two Tier EMail Design

As shown in Figure 8-1, the standard design uses an internal mail server and an external mail server. This design is most appropriate for midsize organizations based on the amount of server resources utilized. Smaller organizations can use this as well, or they might elect to host their e-mail service at an Internet service provider (ISP).

Figure 8-1. Standard Two-Tier E-Mail Design

Figure 8-1. Standard Two-Tier E-Mail Design

As shown in this design, Simple Mail Transfer Protocol (SMTP) servers outside the organization send mail to the external SMTP server, which in turn routes the messages to the internal mail server. This external server is also configured to allow the internal mail servers to route messages through it by using SMTP. In this way, the external mail server is the source of all internal and external mail for the organization. SMTP can be blocked at the firewall to prevent any other systems from sending mail using SMTP.

The internal mail server must perform two functions as well. First, it allows intraorganization mail to route between systems. This allows two users on the internal network to talk to one another without involving the external server. This way, even if the external server is compromised, the attacker is unable to read messages within the organization without compromising the internal mail server as well. Second, this server forwards traffic destined for outside the organization to route through the external SMTP server. This internal server is typically running SMTP and Post Office Protocol 3 (POP3)/Internet Message Access Protocol (IMAP).

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