Description of problem:

When besides amavisd-new also nmh is installed, the default amavisd.conf rule for Avast antivirus lets amavisd discard *any* incoming mail with:

    Blocked INFECTED () {DiscardedInbound,Quarantined}

This is because /bin/scan is called, which is part of nmh (as /usr/bin/scan).

Version-Release number of selected component (if applicable):

Any I know of.  I found the problem using EPEL 7 on CentOS 7, but the amavisd-new and nmh packages conflict everywhere the same way.

Solution:

Comment out the this line in /etc/amavisd/amavisd.conf:

    ['avast! Antivirus', '/bin/scan', '{}', [0], [1], qr/\t(.+)/m ],

Because the use of the Avast virusscanner is probably not very common, it would IMHO be a good idea to comment out this conflicting rule by default.  It costed me quite some time to find out why all my mails were discarded.