Comment 1 for bug 1833331

>The 10mail script is trying to send messages with /usr/bin/mail (aka mailx), which is a user agent, not intended for automated message sending.

'mail' and its various variants are user agents, but also intended for automated message sending. This is the case since the early (pre-Linux) days.

POSIX and its predecessor say:
"Send Mode can be used by applications or users to send messages from the text in standard input."
See:
http://pubs.opengroup.org/onlinepubs/007908799/xcu/mailx.html
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/mailx.html

If no script (like smartd-runner) is specified with '-M exec' directive, smartd also uses 'mail' as a default (see smartd_warning.sh):

mail -s "$SMARTD_SUBJECT" $SMARTD_ADDRESS <<EOF
$SMARTD_FULLMESSAGE
EOF