Block 3rd parties sending through Exchange 2010

I've just been handed a list of accounts that have been sending through my exchange 2010 server.
I Suspect that this is all SPAM bots. How can I block these from going through our server?
I have been told that we do not have a open relay but im not 100% sure after seeing this.

We do not have a edge server or forefront.

Our anti spam solution is provided by Trend hosted services.
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RediersEUS Technology LeadCommented:
If you have domain account sending spam...
Then they have mallware on their machines spreading email
Then you just need to block those accounts from sending email.

That doesn't mean you have an open relay SMTP...
Madan SharmaConsultantCommented:
you can check your server for an open relay on
And also make sure the IP which is used for sending mail should not be used for browsing etc.
I will preferred Microsoft Forefront Security for exchange but anyways if you are using Trend Services make sure that these services are running and configured properly.
KenziiAuthor Commented:
Ok it appears the account sending out the spam is not one setup on anyones machine.
i think that someone is just using it as a sending address. But to make sure.

is there a way i can disable this accuont from sending email but still able to recieve.
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Madan SharmaConsultantCommented:
there is only one way and that is you have to secure your server so that noone can use your server for spaming etc.
KenziiAuthor Commented:
Just checked and i dont have a open relay on the server.
Madan SharmaConsultantCommented:
did you check your IP against blacklisting ?? Also check for trend security solutions is it working and configured properly..
KenziiAuthor Commented:
blacklisting against IP is fine.

the emails do not route through the hosted trend services on the way back. Only incoming mail will route through trend.
We use a smart host to route through our ISP for external outgoing mail.
Madan SharmaConsultantCommented:
So I think you have shared internet connectivity with users and exchange server. am I right ?
You need to install anti spam on your exchange server to avoid this issue..
KenziiAuthor Commented:
Yes thats correct shared internet connection between desktops and exchange server.

Madan SharmaConsultantCommented:
As I have already mention IP for server and internet browsing should be separated otherwise you have to face these kind of issues. Also install anti-spam solution on your exchange server.

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KenziiAuthor Commented:
SPAM Solution resolved issue.
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