What I meant was to try to put 0/3 or 0/2 or even 0/1 in the minute field, if the control panel does not like '*'. As Adam has pointed out you can "smuggle" many things through the command for the crontab, if you direly need this.
But, I repeat, running cron every 5 min will not have any noticeable negative effects other than Moodle taking that much time to react to certain actions like sending forum mails - unless your Moodle site is for a big institution. Well, then you won't be running it in a shared hosting.

Yeah, the "emails are not being sent" is a completely different issue, if the cron is running, that is. You should have started a new discussion on that.