Today we came across an issue on one of our servers and like to share the solution with everyone,
This issue was related to a Cloudlinux 5.0 server with cPanel/WHM version 56 build 24 using CageFS

Several clients on the server started to complain that each mail they received was sending back an error to the sender like the following:

pipe to |/usr/local/cpanel/bin/autorespond [email protected] /home/userdirectory/.autorespond
generated by [email protected]
local delivery failed

Each account had in common they where all running on the same server and all had once configured an autoresponder (active or inactive) for the email account in question.

For some reason, proxyexecd was not running on the server, so all proxy commands like sendmail (/etc/cagefs/proxy.commands) didn't work for users in cage :
# /etc/init.d/proxyexecd status
proxyexec is stopped

After restarting the proxyexecd the issue was resolved for all clients.