This space is consumed by mail notifications
you can check it by typing
sudo find / -type f -size +1000M -exec ls -lh {} \;
It will show large folders above 1000MB
Result will have a folder
/var/mail/username
You can free that space by running the following command
> /var/mail/username
Note that greater than (>) symbol is not a prompt, you have to run the cmd with it.
Now check you space free space by
df -h
Now you have enough free space, Enjoy... :)