Nimda is designed to attack Windows machines by exploiting security holes in Windows. Any infected mail sent to your mail server will be received, but as it is Linux-based, you do not have much to worry about with Nimda on the server. Of course, your mail users may be at risk from this or any other virus -- it does not make any difference what Operating System the server is running if the workstation is running an OS that can be attacked.
There are a number of Linux virus scanners available. You can search on Google for them. One you may wish to investigate is at www.amavis.org. Linux is not inherently virus-proof -- there are many Linux worms and viruses known, despite the claims of Linux promoters.
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This was first published in November 2002