Bug#881696:

from courier's changelog: * courier/filters/courierfilter: Move courierfilter from sbindir to libexecdir. Replace courierfilter is a shell script wrapper that resets the environment before running the real courierfilter.

Bug#881696:

any news on this? tbh, it looks like the courier package isn't really maintained anymore, i even waited 3 months before upgrading to 0.78 and yet it broke my mail server once again: "432 Mail filters temporarily unavailable." \o/