all virus scanners failed

You may want to check to see if ClamAV is running properly, they recently marked 0.92 as End of Life, and that broke alot of things.

Try this:

/etc/init.d/clamav restart

service clamav restart if your on debian. That is, I'm assuming your running clamav. Otherwise, your going to need a mailscanner. Amavis manages spamassassin and clamav, clamav scans for viruses, spamassassin deletes spam.

You may want to have a look at http://workaround.org/ispmail