Sendmail how to and tips

Its status, its log files, how to setup and remove it, the SMTP use, the possible issues :

Sendmail status :

Check if sendmail is running.
Smmsp user.

Sendmail log :

Check log of sendmail.
Where are the logs of sendmail.

Setup and remove :

Remove sendmail.

Sendmail and SMTP :

Setup an SMTP email.
WordPress contact form mail with SMTP email.

Issues :

Port 25 already in use (after the installation of Exim4, it can be sendmail or postfix).
Sendmail keeps running.
Setting the unprivileged uid to 0 is not recommended.
Sendmail still using port 25.