The
list of blocked spammers is at the end of this example file. Note that the address to block can be a
complete e-mail address, a full hostname, a domain only, an IP address, or a subnet.
227
Securing Linux 6
To block a particular e-mail address or host from mailbombing you, log in to your system as root,
edit the /etc/mail/access file, and add a line to DISCARD mail from the offending sender.
After saving the file and exiting the editor, you must convert the access file into a hash-indexed
database called access.db. The database is updated automatically the next time sendmail starts. On
Fedora and other Red Hat systems, you can convert the database immediately, as follows:
# cd /etc/mail
# make
Sendmail should now discard e-mail from the addresses you added.
Spam Relaying
Your e-mail services can also be abused by having your system used as a spam relay. Spam refers to
the unsolicited junk e-mail that has become a common occurrence on the Internet. Relay refers to the
mail server feature that causes it to send mail it receives to another server. (Normally, only users
with valid e-mail accounts on the server are allowed to use a mail server to relay messages on their
behalf. A mail server configured as an open relay will allow anyone to forward e-mail messages
through it and is, therefore, considered to be a very bad practice.
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