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====== Ubuntu - Email - Test Open Relay ======

An outside individual who uses your mail server to deliver email using a false email address is considered relaying mail via your server.  A mail server that allows relaying is usually considered to be setup incorrectly and is frequently abused by spammers.  These spammers find and use unsecured mail servers to send out unsolicited commercial email.

Tracking down a spammer who uses mail servers open to relaying is difficult.  This is because the email appears to be coming from your server, rather than from the original sender.  Such spam being delivered by your mail server can give your company a bad reputation.

----

===== Email Relay Test =====

How do you check your server for relaying?  Easy, just use a computer outside of your organization and type the commands included in the tables below – you’ll want to do this from a command prompt.

In the following examples, mail.example.com is the mail server you are checking, sender@example.com is a valid email account at mail.example.com (or a fake email address – try both), and youremail@outsideaddress.com is the email account you want this message to go to.

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===== An example of a mail server that does NOT allow relaying =====

You type this text:	

<code bash>
telnet mail.example.com 25
</code>

The server should respond with:

<code bash>
Trying 10.10.10.1.
Connected to mail.example.com.

Escape character is ‘^]’.
220 mail.example.com
</code>

You should type:

<code bash>
HELO mail.example
</code>

The server should respond with:

<code>
250 OK
</code>

You should enter:

<code bash>
MAIL FROM:<sender@example.com>
</code>

The server should respond with:

<code bash>
250 OK – Mail from <sender@example.com>
</code>

You should enter:

<code bash>
RCPT TO:<youremail@outsideaddress.com>
</code>

The server should respond with:

<code bash>
550 Relaying is prohibited
</code>

You should enter:

<code bash>
QUIT
</code>

The server should reply with:

<code bash>
221 Closing connect, good bye
</code>

----

===== An example of a mail server that DOES allow relaying =====

You type this text:

<code bash>
telnet mail.example.com 25
</code>

Server should respond with:

<code bash>
Trying 10.10.10.1.
Connected to mail.example.com.
Escape character is ‘^]’.
220 mail.example.com
</code>

You should enter:

<code bash>
HELO mail.example
</code>

The server should respond with:

<code bash>
250 OK
</code>

You should enter:

<code bash>
MAIL FROM:<sender@example.com>
</code>

The server should respond with:

<code bash>
250 OK – Mail from <sender@example.com>
</code>

You should enter:

<code bash>
RCPT TO:<youremail@outsideaddress.com>
</code>

The server should respond with:

<code bash>
250 OK
</code>

You should enter:

<code bash>
DATA
</code>

The server should respond with:

<code bash>
354 End data with <CR><LF><CR><LF>
</code>

You should enter:

<code bash>
From: sender@example.com
To: youremail@outsideaddress.com

Subject: Relay test

This is a relay test and only a test.
(type  <CR><LF>.<CR><LF> or [enter].[enter] to end data)
</code>

The server should respond:

<code bash>
250 OK: Queued as T22122A5
</code>

You should enter:

<code bash>
QUIT
</code>

The server should respond:

<code bash>
221 Closing connect, good bye
</code>

----
 
===== Preventing message relaying with MS Exchange =====

Before you start, check which version you are running – you must be running Microsoft Exchange Server 5.5 or greater, then follow these 7 steps.

  - Go to the Internet Mail Service Properties dialog box in Microsoft Exchange
  - Select the Routing tab at the top.
  - Select the option Reroute incoming SMTP mail (required for POP3/IMAP4 support).
  - Reroute incoming SMTP mail.
  - For each domain you host, you need an entry in the Routing section.
  - Click the Routing Restrictions button.
  - Make sure Hosts and clients with these IP addresses is checked. Leave the list of IP addresses blank.

----

===== Manually testing all relay types =====

You can use the steps outlined above but substitute the “from:” and “to:” below to more thoroughly check your server for different types of relay.  If any of these tests do not give an error than your server is likely open for that type of relay.

==== Standard relay ====

mail from: <nobody@third-party.bar>
rcpt to: <nobody@third-party.bar>

==== Sender domain is long ====

mail from: <test@maps1.pa.vix.com>
rcpt to: <nobody@third-party.bar>

==== Sender’s domain is localhost ====

mail from: <test@localhost>
rcpt to: <nobody@third-party.bar>

==== Sender address without domain (a local user) ====

mail from: <test>

==== Sender address is null (like from Mailer-Daemon) ====

mail from: <>
rcpt to: <nobody@third-party.bar>

==== Sender address uses local hostname ====

mail from: <test@your.local.foo>
rcpt to: <nobody@third-party.bar>

==== Sender address using literal domain address (of local host) ====

mail from: <test@[1.1.1.1]>
rcpt to: <nobody@third-party.bar>

==== Sender address uses local hostname and recipient uses percent hack ====

mail from: <test@your.local.foo>
rcpt to: <nobody%third-party.bar@your.local.foo>

==== Sender address uses local hostname and recipient uses percent hack with domain literal ====

mail from: <test@your.local.foo>
rcpt to: <nobody%third-party.bar@[1.1.1.1]>

==== Quoted ====

mail from: <test@your.local.foo>
rcpt to: <”nobody@third-party.bar”>

==== Quoted string and percent hack ====

mail from: <test@your.local.foo>
rcpt to: <”nobody%third-party.bar”>

==== Quoted string and percent hack and domain literal “x@y”[IP] arbitrary non-local, source-routed destinations ====
RCPT TO:<test@some.other.site@some.site>

==== Sender uses IP rather than FQDN ====

mail from: <test@[1.1.1.1]>
rcpt to: <nobody@third-party.bar@your.local.foo>

==== Recipient uses IP rather than FQDN ====

mail from: <test@your.local.foo>
rcpt to: <”nobody@third-party.bar”@[1.1.1.1]>

==== Recipient uses both FQDN and IP ====

mail from: <test@your.local.foo>
rcpt to: <nobody@third-party.bar@[1.1.1.1]>

==== Sender uses domain literal and recipient uses source routing ====

mail from: <test@[1.1.1.1]>
rcpt to: <@your.local.foo:nobody@third-party.bar>

==== Sender uses local domain and recipient uses source routing with a domain literal ====

mail from: <test@your.local.foo>
rcpt to: <@[1.1.1.1]:nobody@third-party.bar>

==== Recipient address uses UUCP-like destination (“bang path”) ====

mail from: <test@[1.1.1.1]>
rcpt to: <third-party.bar!nobody>

==== Reciepient uses UUCP-like destination and IP ====

mail from: <test@your.local.foo>
rcpt to: <third-party.bar!nobody@[1.1.1.1]>

==== Sender is a known local user from local host; and some mail servers are configured to always allow mail from “postmaster” ====

mail from: <postmaster@your.local.foo>
rcpt to: <nobody@third-party.bar>