Sorry for necromancing this 4 yo thread, but I got exactly the same behavior.
Subject: Cron <smmsp@docker> test -x /etc/init.d/sendmail && test -x /usr/share/sendmail/sendmail && test -x /usr/libexec/sendmail/sendmail && /usr/share/sendmail/sendmail cron-msp
coming from the Host mailcow is running, every 20 miniutes since i installed mailcow. The destination is even from the mailcow internal ipv4 network.