Mine works with
MAILTO=root
but RKH has a place for this in the config file and it works
Code:
#
# Email a message to this address if a warning is found when the
# system is being checked. Multiple addresses may be specified
# simply be separating them with a space. Setting this option to
# null disables the option.
#
# NOTE: This option should be present in the configuration file.
#
#MAIL-ON-WARNING=me@mydomain root@mydomain
MAIL-ON-WARNING=""
#
# Specify the mail command to use if MAIL-ON-WARNING is set.
# NOTE: Double quotes are not required around the command, but
# are required around the subject line if it contains spaces.
#
MAIL_CMD=mail -s "[rkhunter] Warnings found for ${HOST_NAME}"