How to fix amvavis reporting "permission denied" for clamav

A working solution is to change

AllowSupplementaryGroups false -> true

in /etc/clamav/clamd.conf

restart clamav-daemon

While the clamd user was a member of the amavis group, the clamd process was running as the clamav (not clamd) user.

Adding clamav to the amavis group solved the issue.

As AllowSupplementaryGroups does not exist anymore, I ended by adding each user to the other group, like this:

usermod -a -G clamav amavis
usermod -a -G amavis clamav

(and restart everything just to be sure)

/etc/init.d/clamav-daemon restart
/etc/init.d/amavis restart
/etc/init.d/postfix restart

With this, no need to change folder permission on /var/lib/amavis/tmp (and I'm not sure those permissions will be left as is on the next apt-get upgrade...)