Crontab won't run script

[Petrocelli08]

Hi All
I have a strange problem

I want to backup a directory on a FreeBSD mail server. I set up a script which runs the mount_smbfs command. It mounts the remote windows share, tars up the directory, copies it to the share and then unmounts the share. In I run it from the command line it works like so ./backup.sh or /usr/local/etc/backup.sh
When I it runs from /etc/crontab, I get this error below:

mount_smbfs: unable to open connection: syserr = Authentication error
umount: /home/backup: not a file system root directory

Any Ideas as to why this could be happening?