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Disable Exchange 2016 Internal Relay

Posted on 2016-08-14
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Last Modified: 2016-08-17
Recently I setup an Exchange 2016 Server. Everything looks fine except the Exchange 2016 default Receive connector allows internal relay.

Eg: Two emails eric@abc.com and andrew@abc.com in my domain abc.com. An anonymous user can send emails to andrew@abc.com on behalf of eric@abc.com (no authentication required). But if anonymous user try to send email to jason@condoso.com using eric@abc.com, Exchange will refuse to send it.

May I know how could I do to restrict user authentication even for emails in the internal domain? I cannot disable the Anonymous users otherwise Exchange server will not receive any emails.
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Question by:David_zu
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LVL 37

Expert Comment

by:Neil Russell
ID: 41755554
Internal connector should never need anon access.
Emails from the Internet do not come via the internal connector.
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LVL 1

Author Comment

by:David_zu
ID: 41756048
After Exchange Setup, there are 5 receive connectors by default. Default Frontend is the one faced to Internet and receive emails via port 25. The security settings are set as default.

Capture.PNG
I tested using SendSMTP tool. The result shows

8/15/2016 4:53:23 PM Connecting to 4.3.2.1.
8/15/2016 4:53:23 PM Connected.
8/15/2016 4:53:23 PM SMTP connection to 4.3.2.1 successful
8/15/2016 4:53:23 PM SSL status: "before/connect initialization"
8/15/2016 4:53:23 PM SSL status: "before/connect initialization"
8/15/2016 4:53:23 PM SSL status: "SSLv3 write client hello A"
8/15/2016 4:53:23 PM SSL status: "SSLv3 read server hello A"
8/15/2016 4:53:23 PM SSL status: "SSLv3 read server certificate A"
8/15/2016 4:53:23 PM SSL status: "SSLv3 read server key exchange A"
8/15/2016 4:53:23 PM SSL status: "SSLv3 read server done A"
8/15/2016 4:53:23 PM SSL status: "SSLv3 write client key exchange A"
8/15/2016 4:53:23 PM SSL status: "SSLv3 write change cipher spec A"
8/15/2016 4:53:23 PM SSL status: "SSLv3 write finished A"
8/15/2016 4:53:23 PM SSL status: "SSLv3 flush data"
8/15/2016 4:53:23 PM SSL status: "SSLv3 read finished A"
8/15/2016 4:53:23 PM SSL status: "SSL negotiation finished successfully"
8/15/2016 4:53:23 PM SSL status: "SSL negotiation finished successfully"
8/15/2016 4:53:23 PM Cipher: name = ECDHE-RSA-AES256-SHA; description = ECDHE-RSA-AES256-SHA    SSLv3 Kx=ECDH     Au=RSA  Enc=AES(256)  Mac=SHA1
; bits = 256; version = TLSv1/SSLv3;
8/15/2016 4:53:23 PM Encoding text
8/15/2016 4:53:23 PM Disconnecting.
8/15/2016 4:53:23 PM SMTP disconnected from 4.3.2.1
8/15/2016 4:53:23 PM Disconnected.
8/15/2016 4:53:23 PM Message sent successfully.

I expected result shall be like this:

8/15/2016 4:58:17 PM Resolving hostname smtp.office365.com.
8/15/2016 4:58:17 PM Connecting to 132.245.41.98.
8/15/2016 4:58:17 PM Connected.
8/15/2016 4:58:17 PM SMTP connection to smtp.office365.com successful
8/15/2016 4:58:17 PM SSL status: "before/connect initialization"
8/15/2016 4:58:17 PM SSL status: "before/connect initialization"
8/15/2016 4:58:17 PM SSL status: "SSLv3 write client hello A"
8/15/2016 4:58:17 PM SSL status: "SSLv3 read server hello A"
8/15/2016 4:58:17 PM SSL status: "SSLv3 read server certificate A"
8/15/2016 4:58:17 PM SSL status: "SSLv3 read server key exchange A"
8/15/2016 4:58:17 PM SSL status: "SSLv3 read server certificate request A"
8/15/2016 4:58:17 PM SSL status: "SSLv3 read server done A"
8/15/2016 4:58:17 PM SSL status: "SSLv3 write client certificate A"
8/15/2016 4:58:17 PM SSL status: "SSLv3 write client key exchange A"
8/15/2016 4:58:17 PM SSL status: "SSLv3 write change cipher spec A"
8/15/2016 4:58:17 PM SSL status: "SSLv3 write finished A"
8/15/2016 4:58:17 PM SSL status: "SSLv3 flush data"
8/15/2016 4:58:17 PM SSL status: "SSLv3 read finished A"
8/15/2016 4:58:17 PM SSL status: "SSL negotiation finished successfully"
8/15/2016 4:58:17 PM SSL status: "SSL negotiation finished successfully"
8/15/2016 4:58:17 PM Cipher: name = ECDHE-RSA-AES256-SHA; description = ECDHE-RSA-AES256-SHA    SSLv3 Kx=ECDH     Au=RSA  Enc=AES(256)  Mac=SHA1
; bits = 256; version = TLSv1/SSLv3;
8/15/2016 4:58:22 PM Exception: EIdSMTPReplyError SMTP; Client was not authenticated to send anonymous mail during MAIL FROM.
8/15/2016 4:58:22 PM Exception: The operation completed successfully
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LVL 1

Author Comment

by:David_zu
ID: 41756315
In two test, I pretend one domain user, send email to another user in the same domain without authentication.

The first log is the current config, The email sent successfully.

The second log is tested in office365 domain, the email is rejected.

I need to configure the system so it will work as second test.
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LVL 37

Expert Comment

by:Neil Russell
ID: 41756599
Why on earth would you want to allow internal users to Sen I authenticated? Using what client to send?
This would be a big security risk and a daft thing to do to be honest without a Very very good reason.
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LVL 1

Author Comment

by:David_zu
ID: 41756651
Yes I agree. I don't want user to send emails without authentication, no matter it is send to external domain or internal domain. But after I configured the Exchange, it is the behaviour by default. I need to know how to stop it.

Currently Port 587 has no issue. All sender need to be authenticated. But port 25, senders doesn't need to authenticated, if From and To address are in same hosted domain (I want to stop it). I cannot simply untick "Anonymous users" otherwise no email will go into the server.
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LVL 37

Accepted Solution

by:
Neil Russell earned 500 total points
ID: 41756723
Your standard internal receive connector should, assuming you use outlook clients Only accept authenticated MAPI connections. Why would you have port 25 on an internal connector?
The external connector should never be accepting emails that are from your internal users.  IF it does then you become a spam trap and will have millions of emails on your server as soon as it is found out and trust me, that will not take long nowadays.

External connectors are for just that, EXTERNAL emails NOT emails from inside your organisation.  the test you did is a good one and proved that your configuration is CORRECT.
Of course the best way to protect it to have an "Edge Transport Server" set up and then you can have a simple reject rule on it if an email comes into your domain with a From: address that is xxxxx@Yourdomain.com
You should never expect to see inbound smtp traffic from one of your own internal accounts.
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LVL 1

Author Comment

by:David_zu
ID: 41757241
Port 25 is bind to Front End Transport role, which I believe it shall be the external connector. "The external connectors shall never be accepting emails that are from your internal users". This is exactly what I want but I cannot find the way to do. I cannot find from web UI. If it shall be done from PowerShell please help to provide the command of PowerShell. It is a single Exchange server deployment.
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LVL 1

Author Comment

by:David_zu
ID: 41758984
I setup the Edge server and so far this shall be the only way to block unauthorised spam mail. I still cannot find a proper way to block the unauthorised emails without Edge server.
0
 
LVL 1

Author Closing Comment

by:David_zu
ID: 41758985
The Edge Transport server so far is the only way to block unauthorised emails.
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