PMDF System Manager's Guide
30.1 What is an e-mail Firewall?
Here an e-mail firewall refers to an enhanced,
firewall-oriented e-mail handling component on an Internet firewall
system. A basic Internet firewall system generally controls what TCP/IP
interactions are allowed between the external world, considered to be
unsafe, and an internal, protected environment, considered to be safe.
To be an e-mail firewall system, this system should also check and
control the e-mail passing between the internal and external
- An e-mail firewall can perform address transformations, converting
external presentation addresses in messages incoming from the external
world to actual internal addresses, and transforming internal addresses
to external presentation addresses on messages outgoing to the external
world. See Chapter 3 for a discussion of centralized naming in
general, and Section 188.8.131.52 below for mention of special considerations
on an e-mail firewall.
- An e-mail firewall can enforce restrictions on what messages are
allowed in or out. See Section 30.4.5 below. In particular, an e-mail
firewall can disallow certain sorts of message traffic, and can be
configured to protect against denial of service attacks.
- An e-mail firewall can be set up to perform filtering on message
content, e.g., limiting message size, or checking incoming
binary attachments for viruses. See Section 30.4.7 below.
- An e-mail firewall is careful in what information it emits in
response to external systems' possible probe attempts. See
Section 30.4.8 below.
- And an e-mail firewall provides facilities for message logging and
message traffic statistics. See Section 30.4.3.