Via command line program and e. g. procmail (PostMaster.pl)

OTRS is able to receive emails via a command line programm (bin/PostMaster.pl).

That means emails will be shown in your OTRS system if the MDA (mail delivery agent, e. g. procmail) pipes the emails into bin/PostMaster.pl.

To test the bin/PostMaster.pl on your command line (without MDA) use:
shell:~ # cat /opt/otrs/doc/test-email-1.box | /opt/otrs/bin/PostMaster.pl shell:~ #
If the email is shown in the QueueView then your setup works fine.

Procmail is a very common e-mail filter in the Linux enviroment. It will be probably installed on your system. If not have a look at the procmail homepage.

To configure procmail for that (requires a procmail configured MTA (e. g. sendmail, postfix, exim or qmail)) use the ~otrs/.procmailrc and modify/add the following.
SYS_HOME=$HOME PATH=/bin:/usr/bin:/usr/local/bin # -- # Pipe all email into the PostMaster process. # -- :0 : | $SYS_HOME/bin/PostMaster.pl
All emails sent to the local otrs user will be piped into bin/PostMaster.pl and then shown in your QueueView.

Fetching emails via POP3 or IMAP and fetchmail for PostMaster.pl

In order to get e-mails from your mail server via a POP3 or IMAP mailbox to the OTRS machine/local otrs account and to procmail use fetchmail. Note: A working SMTP configuration on the OTRS machine is a condition.

Example 7-1. .fetchmailrc

#poll (mailserver) protocol POP3 user (user) password (password) is (localuser)
poll mail.example.com protocol POP3 user joe password mama is otrs

Don't forget to set the .fetchmailrc to 710 ("chmod 710 .fetchmailrc")!

So if "fetchmail -a" is executed (maybe via cron), all e-mails will be forwarded to the local otrs account.