man 5 crontab resolverá seu problema. Diz, em parte:
An active line in a crontab will be either an environment setting or a
cron command. An environment setting is of the form,
name = value
where the spaces around the equal-sign (=) are optional, and any
subsequent non-leading spaces in value will be part of the value
assigned to name. The value string may be placed in quotes (single or
double, but matching) to preserve leading or trailing blanks.
Several environment variables are set up automatically by the
bcron-exec(8) program. SHELL is set to /bin/sh, and LOGNAME, USER, and
HOME are set from the /etc/passwd line of the crontab's owner.
In addition to LOGNAME, USER, HOME, and SHELL, bcron-exec(8) will look
at MAILTO if it has any reason to send mail as a result of running
commands in ''this'' crontab. If MAILTO is defined (and non-empty),
mail is sent to the user so named. If MAILTO is defined but empty
(MAILTO=""), no mail will be sent. Otherwise mail is sent to the owner
of the crontab. This option is useful if you decide on /bin/mail
instead of /usr/lib/sendmail as your mailer when you install cron --
/bin/mail doesn't do aliasing, and UUCP usually doesn't read its mail.