I see lines like this in dmesg:

set{u,g}id pid 4131 (sendmail) was invoked by uid 0 ppid 1 (init) with fd 1,2 closed
set{u,g}id pid 27186 (sendmail) was invoked by uid 0 ppid 1 (init) with fd 1,2 closed
set{u,g}id pid 2577 (sendmail) was invoked by uid 0 ppid 1 (init) with fd 1,2 closed
set{u,g}id pid 27183 (sendmail) was invoked by uid 0 ppid 1 (init) with fd 1,2 closed

Seems like it is related to cron activities. How can I avoid them?

-- 
..уйдет в /dev/null все. Включая и сам /dev/null.