There's lots of
sh: 1: /usr/sbin/sendmail: not found in my apache2 error.log, the problem is, I don't recall any of my site/apps trying to send mails or whatever, and I've already installed WP Mail SMTP on my wordpress blogs, which works all right, so my problem is, how do I find out which app or site is trying to do this all the time?
Edit: I did find interesting lines in /var/log/mail.log:
Jun 22 07:27:31 sm-mta: r5H8U4O1014238: to=<firstname.lastname@example.org>, ctladdr=<www-data@email@example.com> (33/33), delay=4+22:57:27, xdelay=00:00:01, mailer=esmtp, pri=63391559, relay=mx00.1and1.com. [18.104.22.168], dsn=4.0.0, stat=Deferred: 421 invalid sender domain.
I've already followed instructions to completely uninstall any mailer apps on my server like this:
apt-get remove sendmail sendmail-bin postfix apt-get purge postfix exim4 sendmail sendmail-bin
Those annoying lines are still showing up every now and then, what do I do now?