Emergency - email virus issue

One of my users started receiving hundreds of emails with failed delivery reports of an email he never sent himself - it contains a zip file attachment, and inside the zip file is an executable, definitely bogus.

How do I make sure this is terminated? He changed his password immediately. It is odd that nothing shows up in his sent items folder either. I'm not sure how these types of email account hijacking viruses work, so am hoping for someone more knowledgeable to pipe in!

He is starting to receive all sorts of emails back from people notifying him of the spam email. I checked mail flow on my exchange server and can see all of the sent emails. How do I put a stop to this??
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CoSmismgrAsked:
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CoSmismgrAuthor Commented:
It appears no more of the emails have been sent since the password change.
CoSmismgrAuthor Commented:
Still would like to know what caused this, I cant be sure it wont happen again until I know
Electronic OfficeEngineerCommented:
I saw this recently. I believe what happened is that someone was able to hack the user's password and then use the relay function of our onsite email server to bounce email to thousands of email addresses. The email does not originate from the user but does get addressed as if it were sent by them. In our case I was able to adjust relay settings on the exchange server to only allow relays from the server, not for anywhere else on our network or elsewhere.

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JohnBusiness Consultant (Owner)Commented:
I have also seen this before, and the cause was someone hacking into the web mail site. Changing the password to a secure password stopped the problem and in the case I reference, the problem did not come back.

Be sure you use a very secure password (with special characters).
CoSmismgrAuthor Commented:
Thanks guys, I am certain it was hacked credentials and am implementing a more secure password policy.
JohnBusiness Consultant (Owner)Commented:
@CoSmismgr  - Thanks for the update and I was happy to help.
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