There are lot of information you haven't provided in your post.
1. Is /var/mail a separate filesystem or embedded in /var?
2. Disk structure (Output of `du -h` & `fdisk -l`)
3. Is /var currently on Logical volume ?
4. If yes, provide output of lvdisplay & vgdisplay.
5. Where exactly you are failing ? Creation? Extension?
6. Error messages.
Refer to below post:
http://www.linuxquestions.org/questi...tition-695815/