All you need to do is create a different user and set a Password for the user.
click on SYSTEM / ADMINISTRATION / USERS AND GROUPS
Make the new user have the same username as your email address
and setup the Cron under this new user.
(Username would be donotreply)
Then if the Hostname is your domain name which you want it to be then the email the cronjob sends will look like its from that address
So simple yet it took me ages to figure it out. I feel like an idiot. Anyway hope this helps anyone else trying to do the same thing with a Cronjob.
For newbies wondering how to become that new user, simply open a shell and type this:
"su donotreply" (donotreply being the new user you have created of course)
Enter Password and then type "crontab -e" and setup the cron as the new user.