Block External Users sending to Internal Distribution List

Bogart Bogart
Bogart Bogart used Ask the Experts™
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Hi Experts,

I have set our Exchange 2010 SP3 to prevent external users to send to our internal Distribution List. In fact, the default is to Require that all senders are authenticated under Message Delivery Restrictions. However, this isn't working at the moment. I also have tried to create Transport Rules with the condition for messages outside the organization destined to members of a certain DL forward to a moderator. This works fine if the email is emailed directly to the DL email address itself but if the email is sent directly to the email address of an individual user it will be quarantined for approval which is not really what we want. Any ideas what's causing this?
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AkhaterSolutions Architect

Commented:
what are you trying to accomplish exactly ? it seems to me that everything is working fine

1) you send from outside to the email address of the DL the email gets rejects (that's what should happen)

2) you created a transport rule to ask for moderation if an email is sent from outside the org to a user in a specific DL and this is what is happening no ?


" Require that all senders are authenticated" means that emails sent TO THE DL will not get accepted and not to its members . if you want no one to send an email to the members change your transport rule to

1) email sender is form outside the organization
2) recipient is member of specific DL
3) Reject the message with the enhanced status code


that should cover you

Author

Commented:
The "Require that all senders are authenticated" isn't working which means external emails are getting through if an external user sends to the DL itself.

Setting up a transport rule it seems that it is working fine like you've oveserved BUT just would like to block the DL@test.com and not on the user level email address i.e. user1@test.com
AkhaterSolutions Architect

Commented:
when you say "external emails" you means emails sent from external users like hotmail.com right ?
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Author

Commented:
Hi Akhater, Yes external users such as Hotmail, yahoo etc.

The "Require that all senders......" should just do the job by blocking incoming emails directly destined to the DL email address but still allows emails send individually to internal emails.
AkhaterSolutions Architect

Commented:
Can you test with a new dl? Create a brand new dl don't change anything in it's config, add to it a couple of users and email the dl from Hotmail / yahoo

Author

Commented:
Hi Akhater, I have done this already and the same is happening.
AkhaterSolutions Architect

Commented:
Oh well then please check your receive connectors is it configured with "externally secured"?

Author

Commented:
I have tried that setting as well to no avail. Weird that it just doesn't want to work!
AkhaterSolutions Architect

Commented:
Which setting? No you should NOT have externally secured on your receive connector

Author

Commented:
I don't have it.

Author

Commented:
I should have indicated that on my Default connector I don't have that (Externally Secured) selected as I don't want my server to become a relay.

Looking at the connectors it allows incoming emails alright but the behaviour of not allowing unauthenticated (external) users to send to internal DL isn't working which is really the problem.
AkhaterSolutions Architect

Commented:
Can you share the output of

Get-ReceiveConnector | fl name,*authmec*

Author

Commented:
Name          : Default XXXXXXXXXXXXXX
AuthMechanism : Tls, Integrated, BasicAuth, BasicAuthRequireTLS, ExchangeServer

Name          : Client XXXXXXXXXXXXXX
AuthMechanism : Tls, Integrated, BasicAuth, BasicAuthRequireTLS

Name          : Application Relay
AuthMechanism : Tls, ExternalAuthoritative
Solutions Architect
Commented:
OK so we have "Application Relay" that has "ExternalAuthoritative" enabled

how do you receive emails from outside ?
you have an anti-spam / relay that delivers emails to Exchange ?


Get-ReceiveConnector "Application Relay" | fl name,*authmec*,RemoteIPRanges

what is the output of that ?

Author

Commented:
The application relay only allows internal network to relay to our exchange - this is mainly outgoing. So this is not the issue.

We do have Sophos Virtual Email Filter. Spoken to the guys but was advised that it's got nothing to do with the filter.
AkhaterSolutions Architect

Commented:
Get-ReceiveConnector "Application Relay" | fl name,*authmec*,RemoteIPRanges
please run this and make sure that the SoPhos ip address is not in the RemoteIPRanges

Author

Commented:
I can confirm that Sophos isn't part of the list.
AkhaterSolutions Architect

Commented:
ok can I ask you to create a new Receive connector with just the IP of sophos keep everything default just allow Anonymous on it and test again ?

Author

Commented:
Hi Akhater,

I am tempted to restart the server at this stage before proceeding. I will provide update on any improvements - hopefully.

Author

Commented:
In Exchange 2007 and 2010 having your filter as trusted will bypass authentication.

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