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Part V: Hacking Applications
The final period marks the end of the message. After you enter this final
period, your message will be sent if relaying is allowed.
8. Check for relaying on your server:
• Look for a message similar to Relay not allowed coming back
from the server.
If you get a message similar to this, SMTP relaying is either not
allowed on your server or is being filtered because many servers
block messages that appear to originate from the outside yet come
from the inside.
You might get this message after you enter the rcpt to: command.
• If you don’t receive a message from your server, check your Inbox
for the relayed e-mail.
If you receive the test e-mail you sent, SMTP relaying is enabled on
your server and probably needs to be disabled. The last thing you
want is to let spammers or other attackers make it look like you’re
sending tons of spam, or worse, to be blacklisted by one or more
of the blacklist providers. Ending up on a blacklist can disrupt
e-mail sending and receiving — not good for business!
Countermeasures against SMTP relay attacks
You can implement the following countermeasures on your e-mail server to
disable or at least control SMTP relaying:
✓ Disable SMTP relay on your e-mail server. If you don’t know whether
you need SMTP relay, you probably don’t. You can enable SMTP relay
for specific hosts on the server or within your firewall configuration.
✓ Enforce authentication if your e-mail server allows it. You might be
able to require password authentication on an e-mail address that
matches the e-mail server’s domain. Check your e-mail server and client
documentation for details on setting up this type of authentication.
E-mail header disclosures
If your e-mail client and server are configured with typical defaults, a malicious
attacker might find critical pieces of information:
✓ Internal IP address of your e-mail client machine (which can lead to the
enumeration of your internal network)
✓ Software versions of your client and e-mail server along with their
vulnerabilities
✓ Hostnames that can divulge your network naming conventions