SMTP authentication with Exchange 2010


I've got a config issue with Exchange 2010 : our server is NOT open relay, but accept sending mail from internal domain users ( without authentication. For exemple, sending a mail using Telnet will work as long as the sender (MAIL FROM) is

How configure the Exchange server to prevent this, and require authentication even if the sender is an internal user ?

Christian KAZADIAsked:
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Will SzymkowskiSenior Solution ArchitectCommented:
You need to look at ALL your Receive Connector permissions groups and make sure that none of them have Anonymous selected. If you have specific receive connectors that require sending out email anonymously (like an applicaiton server) you need to lock these does using IP addresses.

Christian KAZADIAuthor Commented:
Thanks Will, but if I remove "anonymous" from the allowed groups on my default Receive connector I cannot receive mail from the outside anymore.
Mail are rejected with SMTP error 530 5.7.1 Client was not authenticated

(I've got 2 receive connectors, one on port 465 which doesn't allow anonymous connexions, and one on 25 port which allow it ; if I remove anonymous on port 25, no more incoming mails...)


Simon Butler (Sembee)ConsultantCommented:
That is the expected behaviour.

The only way to stop it is to block your internal IP address range from accessing the connector that has anonymous enabled on it. If you disable anonymous on all receive connectors then you will not receive any email from the outside world at all!

Christian KAZADIAuthor Commented:
OK, I think there is a little misunderstanding here... My mistake.
I want to prevent an external user to send mail using an internal domain address to an internal domain address without be authenticate (mainly for avoiding fishing)

Look at this example (the internal domain is, i think it will be more clear :

# Telnet 25

EHLO Hello [192.x.x.x]
250 2.1.0 Sender OK
550 5.7.1 Unable to relay  <------------- OK that' what I want to
250 2.1.5 Recipient OK <----------------- IF destination domain is internal, I don't need authentication ! That's what I want to prevent.
354 Start mail input; end with <CRLF>.<CRLF>
250 2.6.0 <fd939a34-4074-41ab-943f-18f9798c3946@SVRCAS1.biac.lan> [InternalId=10063873] Queued mail for delivery
Simon Butler (Sembee)ConsultantCommented:
Spoofing is the most common form of spam sending, and what you are trying to do is one of, if not the favourite spammer's trick.
Exchange cannot stop that on its own - you need to use third party spam filtering software to do that for you. SPF records and the like can help as well, but without additional third party support it cannot be stopped.

Delivery is only controlled by the recipient, not the sender. The sender can be anything.
Even if you could stop it, the way that email is setup means the spammer could put something else in the actual header of the email to look like the message is coming from elsewhere.

This isn't an Exchange problem - all SMTP servers have the issue, and that is simply because of the way SMTP is written.


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