ISP Complaining about spamming

Hello Experts,
I got an email from my ISP that one of the device / computer behind my spamming a lot. If I don't shut it down they will disconnect the services. My Barracuda SPAM Filter is fine, I checked with no flags or unusual activity. I checked on my Cisco ASA5505 and couldn't find anything unusual. How do I go from here to find out the culprit on my network?
TY!
itcsproAsked:
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JohnBusiness Consultant (Owner)Commented:
If you have a small network, use Wire Shark or like packet sniffer to see who is sending out large volumes of data. Try to pick a low time of use so the culprit stands out.
Natty GregIn Theory (IT)Commented:
You need to scan each computer with a malware and AV scanner one of them is sending mail secretly. I had the same issue a while back and found a mail program on my wife's computer that was spamming the crap out of my isp
itcsproAuthor Commented:
That's not a optimum solution if you have more than 100 computers on the network.
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JohnBusiness Consultant (Owner)Commented:
Any one of a million computers could be sending out emails. There is no magic button to pick out the problem computer. You need to check traffic outbound as I suggest and / or check each machine for viruses as was also suggested. It is never optimal to find a needle in a haystack.
Natty GregIn Theory (IT)Commented:
Not an option if you have 100 computers, here is one for you, you may get an ear full for this and it includes work.
Block all ip address via mac-address control then, let them back on the internet one by one, and call the isp after each one or keep them on the phone until you either restore all or run into the machine (s) that are spamming them so for each you ask them for a report.

OR

Use McCafe enterprise AV, where you can scan each computer on the network from one central location.

CONS - if the infected machine is not on when you do this you will not find it.

OR
its time to meet and greet your coworker cause you will have no choice but to scan each computer individually.

after you find the culprit set a group policy to monitor and isolate excessive email sending or use of resources or virus like activities, its been awhile, that would have isolate and  block that computers (s) for you so u wouldn't have to do what you must do right now.
Daniel SheppardSenior Network Analyst - Core & PerimeterCommented:
Since you have a small office and a Barracuda, set your barracuda as your outbound for all your computers (or if you have exchange don't worry about it).

Configure your ASA to block port 25 outbound for all except your barracuda (and exchange if applicable).

My guess, someone has some malware and that is sending email outbound port 25.  This will also allow you to see which computer is sending email outbound port 25 maliciously if you log the denies.

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itcsproAuthor Commented:
Found in first hour during peak hours. Thanks!
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