My exim4 process is configured based on this post and other points noted here.

Today it started failing - clamav failed to read its db (locked) possibly due to freshclam runs.

Debian bug gave the hint - the packages in volatile have this fixed (a non-security update that fixes this issue in stable).

So - added to my apt-config:

deb http://volatile.debian.org/debian-volatile etch/volatile main contrib non-free