IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Communications Remarks
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newly received consignment of spam mail. Appropriate filter lists are
available in the Internet and can be obtained from various manufacturers of
communications software.
Authorised protocols and services on the mail server must be specified. For
example, it is advisable to authorise SMTP (TCP port 25) for outbound and
inbound links, but only authorise POP3 for internal links.
The mail server must be protected against use as a spam relay. For this
purpose, the mail server should be so configured that it only accepts e-mail
intended for the organisation and only transmits e-mail originating from the
staff of the organisation. The mail server should only accept incoming e-mail
if the IP address of the transmitting mail server is located in an IP network
authorised explicitly by the administrator, or if the mail server holds an MX
entry for the recipient. All other e-mail must be rejected with a corresponding
error message.
In spite of these safeguards, authorised users can continue to send/receive email
to/from any required party. However, the filtration of incoming e-mail
described above prevents the mail server from being misused as a spam relay
by external parties.
If IP networks from which e-mail is to be accepted have been inadvertently
omitted from the list mentioned above, the administrator of the mail server
must be informed duly so that he/she can include these networks subsequently
in the list.
If, instead of operating its own mail server, an organisation accesses the mail
server of a provider via one or more mail clients, clarification by the provider
is required as to the rules and security measures applicable on that server
(refer to S 2.123 Selection of a mail provider).
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IT-Baseline Protection Manual: Oktober 2000