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Somebody sent spam using one of my users accounts

I have a simple one domain, one exchange server (2003) system.  One of my 70 users came in the other day to find 500 undeliverable messages in her Outlook.  They were all spam using her account.I checked MXLOOKUP and we do not have an open relay.  There are only a few messages in my System Manager queue.  This is a one-time occurrence.  Should I ignore it and see if it repeats?  Or what should I check to prevent a reoccurrence.  Our incoming mail goes through a spam filter, but I don't believe our outgoing mail does.
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tomcurrier
Asked:
tomcurrier
1 Solution
 
Chris DentPowerShell DeveloperCommented:

Not an uncommon situation unfortunately.

It's likely that the spam didn't originate at your servers at all. Spoofing SMTP addresses is utterly trivial so pretty much anyone can pick up an address and use it as they wish.

You don't get a great deal of choice about what you do with this. Your only option to help reduce the impact should it occur again is to add an SPF (Sender Policy Framework) record for your domain (a TXT Record on your public DNS Server).

The SPF Record allows you to explicitly declare which IP addresses are permitted to send mail for your domain name. It's an extremely useful mechanism for reducing abuse as you've seen above.

The tricky bit is that it is impossible to completely stop abuse like this, SMTP is inherently insecure. Things like SPF sit on top of (or in front of) SMTP and attempt to add a level of security. Not everyone checks SPF Records though. Those that don't will still return NDRs without further consideration. And not all ISPs allow you to add them, an annoying situation.

For instructions (and wizards) for generating SPF Records you can use either this site (this one also contains full details of the syntax):

http://www.openspf.org/

Or this one:

http://www.microsoft.com/mscorp/safety/content/technologies/senderid/wizard/

If you've created yourself a record and aren't sure at all please don't hesitate to post back.

HTH

Chris
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tomcurrierAuthor Commented:
Chris, many thanks.  That helps
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