Cron jobs can have there results returned as mail via the unix mail system. If left unchecked for a while the number of mails can mount up and you probably do not care about the old results. the command d * will delete all mails

$ mail
Mail version 8.1 6/6/93.  Type ? for help.
"/var/mail/lounge": 2726 messages 25 new 2726 unread
? d *
? quit

$ mail
No mail for user