Exchange – Are you an Open Relay?


KB ID 0000087 Dtd 09/11/09


When Email was a new medium, pretty much all mail servers were open relays, and nobody really cared, after all if someone in Nigeria wanted to relay mail through a college server in Manchester then why shouldn’t they? Sadly with the explosion of Email and internet use this is no longer an option, because all those annoying Emails you get for Viagra or insider stock tips have probably been sent to you through an open relay. That’s to say someone like you with a mail server that’s happily sending out someone else’s mail.

Now you may be the sort of person who does not care, if that’s true then "be warned" Major ISP’s and Email handlers are routinely blocking "Suspect" mail servers, you might not even know you had a problem, and the first thing you know about it is,you cant send mail to a particular domain because you are on a blacklist, or worse your ISP cuts off your internet access, or you get an early knock on the door from some nice men in suits wanting to know why 400Gb of "dodgy photos" was sent from your IP address this morning.


Test your Mail Server with Telnet


1. OK – the first thing we need to do is get on a PC that isn’t logged into the domain (Exchange can be set to relay mail from a client that’s authenticated to the domain regardless) – So take that out of the equation by not being in the domain. Now open a good old fashioned command window . Click Start > Run > CMD {enter}
Alert! – To put "Run" on your start menu > Right click your task bar > Properties > Start Menu Tab > Customise > Tick "Run Command" > OK > OK.

2. You now need to connect to your mail server using a telnet command, to do this you need to know either the name or the IP address of the server. the command is,
telnet <IP Address or Server Name> 25 {enter}

Telnet’ is not recognized as an internal or external command

3. The server should respond with a "Banner" this lets you know you connects successfully NOTE some anti virus programs block this (McAfee for example) you need to go to its access protection settings and untick "Prevent mass mailing worms from sending mail". Don’t forget to turn it back on again later ๐Ÿ™‚

4. You are going to send an Email from command line, the first thing you need to do is say hello to the server, though being an exchange server the command is,
ehlo {enter}
What we want to see are 250 messages in our example we got, Hello []

5. Type the following,
again we want to be seeing a 250 message, if you didn’t get one you made a spelling mistake start again ๐Ÿ™
In our example we got,
250 2.1.0….Sender OK

6. Now we are going to attempt to relay mail for a different domain this will tell us if the server is an open relay or not. Type the following,
Note if the Server gives you a message like,
550 5.7.1 Unable to relay for

NOTE if the server responds with
250 2.1.5
Then either you ignored me in step 1 and your in the domain – or YOU ARE AN OPEN RELAY.

Officially there are three things, that if set wrong can leave you as an open relay,

1. Your Default SMTP Virtual Server.

2. Your SMTP Connector.

3. You have ISA server installed and set incorrectly.

Step 1 – Check the SMTP Virtual Server

1. On the Exchange Server Click Start > All Programs > Microsoft Exchange > System Manager.

2. Expand Administrative Groups > First Administrative group > Servers > {your server name} > Protocols > SMTP > Right Click "Default SMTP Virtual Server" > Properties.

3. On the properties window select the Access tab > Click the "Relay" Button.

4. On the "Relay Restrictions" window Check that, "Only the list below" is selected > It’s not unusual (in fact its the default) that the window is empty, you may see the Exchange server IP addresses in here – or in some cases other hosts on your network that have been set up to relay mail – (Backup software that emails you, or SQL servers that email events for example) > And ensure the box at the bottom that says "Allow all computers that successfully authenticate to relay, regardless of the list above" IS TICKED.

Step 2 – Check the SMTP Connector

NOTE: You might not have an Exchange connector don’t panic if its not there ๐Ÿ™‚


1. On the exchange Server Click Start > All Programs > Microsoft Exchange > System Manager.

2. Expand Administrative Groups > First Administrative group > Routing Groups > First Routing Group > Connectors. > {your connector name}
NOTE – you may have many different routing groups and the path in your exchange system manger might be under Servers > {your server name} > Connectors.

3. Right click your connector and select Properties.

4 Select the "Address Space" Tab > You should see the following > Address Type = SMTP > Address = * > Cost = 1 > Connector Scope = "Entire Organisation" > "Allow Messages to be relayed to these domains" IS NOT TICKED.

Step 3 – Check ISA

ISA Server 2000 – had a problem where if you had a mail publishing rule for SMTP it set an open relay -check! Also make sure make sure is NOT in the list of IP addresses that are allowed to relay in the the properties section of the default SMTP Virtual server.

Related Articles, References, Credits, or External Links


Author: Migrated

Share This Post On