Where the sendmail program can be found,
usually /usr/sbin/sendmail or
configure does an honest attempt of
locating this one for you and set a default, but if it fails,
you can set it here.
Systems not using sendmail should set this directive to the
sendmail wrapper/replacement their mail system offers, if any.
For example, Qmail
users can normally set it to
qmail-inject does not require any option to
process mail correctly.
This directive works also under Windows. If set, smtp,
smtp_port and sendmail_from are
ignored and the specified command is executed.