[otrs] Cron jobs for otrs user does not start

Hello, I have set the crontab for the otrs user. When I type "crontab -l -u otrs' I can see all the entries like on page 31 of the "OTRS 2.0 Admin Manual". The problem is that it seems that this crontab is never executed; I must launch "/opt/otrs/bin/PostMasterPOP3.pl" manually in order to have teh new emails comming in OTRS system. This line */10 * * * * $HOME/bin/PostMasterPOP3.pl >> /dev/null exists in the crontab. I don't see were the problem could be. Can anyone help me? Thank you in advance. Alina.