I want to deny external users from using relaying to my Exchnage Server telnet to port 25

Hi. We have Exchnage 2007. I noticed that when i connect to the internet from outside my network and i telnet "exchnage servname" 25 , i can then send an email from anyone from my organisation which is a huge security risk. Please advise how i can deny this
BSTITAsked:
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Rajith EnchiparambilConnect With a Mentor Office 365 & Exchange ArchitectCommented:
No, you can only send emails to users within your own domain.

If the server were operating as an open relay you would be able to send email to any other domain. By default Exchange server does not allow this to happen and you would get an error message after entering the RCPT TO line. If you are able to send to an outside domain, it is a good idea to check your configuration and ensure that the server is not operating as an open relay.

http://www.msexchange.org/articles/Sending-Email-without-Client.html
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Rajith EnchiparambilOffice 365 & Exchange ArchitectCommented:
That is how it works. exchange 2007 is secure by default. You have to go and change the setting to make it an open relay.

When you do a telnet test, can you send an email by choosing, say user@google.com as the sender and user@microsoft.com as the recipient? Then, your server is an open relay.

But, if your sender in telnet is a user in your exchange and the recipient is anyone in the world, telnet will work! All exchange is trying is to send an email from your internal user to someone outside.

Rajith.
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Rajith EnchiparambilOffice 365 & Exchange ArchitectCommented:
If you are so worried, just disable telnet protocol on your external firewall.

Rajith.
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BSTITAuthor Commented:
Does this mean an external user that knows my email address can telnet into my exchange server and send helo and a message using my email address to anyone in the world. If so then thats a secuirty risk
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BSTITAuthor Commented:
I cannot disable telnet because my Firewall guys use this to get in and assist us. What else can i do?
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BSTITAuthor Commented:
What if an attacker gets my email address and sends a mail via telnet to another user on my domain instructing him to transfer money. That will work, how can i deny this?
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danf0xCommented:
You can always change the port from 25 to 587.  I have done this for some of my users as that gets rid of a ton of spambots but any decent antispam software that does sender verification will stop those type messages coming through.
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BSTITAuthor Commented:
I heard there is a way we can specify only internal ip addresses under the send connector, is this correct as this will eliminate the problem and if so how do i do this
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