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Chapter 13: Communication and Messaging Systems
Finally, work with your marketing team and web developers to ensure that
company e-mail addresses are not posted on the web. Also, educate your
users about not doing this.
Relay
SMTP relay lets users send e-mails through external servers. Open e-mail
relays aren’t the problem they used to be, but you still need to check for
them. Spammers and hackers can use an e-mail server to send spam or malware
through e-mail under the guise of the unsuspecting open-relay owner.
Be sure to test for open relay from outside your network. If you test from
inside, you might get a false positive because outbound e-mail relaying might
be configured and necessary for your internal e-mail clients to send messages
to the outside world. However, if a client system is compromised, that
issue could be just what the bad guys need to launch a spamming or malware
attack.
Automatic testing
Here are a couple of easy ways to test your server for SMTP relay:
✓ Free online tools: One of my favorite online tools is located at www.
abuse.net/relay.html.
✓ Windows-based tools: One example is NetScanTools Pro (www.
netscantools.com). You can run an SMTP Relay check on your e-mail
server with NetScanTools Pro, as shown in Figure 13-9.
Although some SMTP servers accept inbound relay connections and
make it look like relaying works, this isn’t always the case because the
initial connection might be allowed, but the filtering actually takes place
behind the scenes. Check whether the e-mail actually made it through
by checking the account you sent the test relay message to.
In NetScanTools Pro, you simply enter values for the SMTP mail server
name, Your Sending Domain Name. Inside Test Message Settings, enter the
Recipient Email Address and Sender’s Email Address.
When the test is complete, simply click View Relay Test Results. Depending
on which option you’ve selected, you’ll see the results of your tests, as
shown in Figure 13-10.
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