Mail program (Thunderbird) starting periodically after editing crontab

I installed Moodle (web based content management system) a couple of days ago. Yesterday I added cron.php in /var/spool/cron/tabs/ ). I also put it into crontab.

0-59 * * * * /usr/bin/php /srv/www/htdocs/moodle/admin/cli/cron.php >/dev/null

Now Thunderbird, the mail program, will restart every 4 min 54 sec and it starts on login.

I have removed the line in crontab and removed the cron job in /var/spool/cron/tabs/ and restarted.

I don’t know how to troubleshoot this.

Any help appreciated.

Solved.
gkrellm was doing a mail check on login an 5 minutes.