By default, sendmail (the Internet mail daemon on Unix and VMS) allows
email to be forwarded to other sites, even if it originates offsite.
This has been exploited by people to propagate junk email (spam),
using our servers to replicate messages and disguise the real origin of
the mail. To prevent abuse, this should be blocked. Patches are
available for sendmail version 8.8, while version 8.9 incorporates
easier-to-use relay rules already, which may need to be uncommented
or otherwise configured. Earlier versions are not easily fixable and should
be upgraded.
You may use the Web script
nph-chkspam
to verify relay status, or just use telnet
to test the simple case (if your machine is set to relay
internal mail, you will have to do this from an external host):
$ telnet node 25
HELO mynode
MAIL FROM: yourname
RCPT TO: <bill@msn.com>
QUIT