"Mail cannot update your mailbox because your home directory is full"

So i get the following message when I try and open mail:

"Mail cannot update your mailbox because your home directory is full: You must free up space in your home folder before using mail. Delete unneeded documents or move documents to another volume"

I have over 5GB in hard disk space left.

Well, you have 5GB but the system uses a fair amount of space as virtual memory. It's recommend to leave around 10% of your drive space free for such occasions.

I'd try to remove some files you don't need and give it another try.
Search around, there are a ton of space saving hints in older forum threads!

Running with 5GB can be frustrating at times. As Aptmunich said, OS X allocates space dynamically for the virtual memory. If it can't find enough, you would see the "Beachball" running more often than normal. I would suggest you to free some space or transfer some of the big files to an external HD.

ok so i got rid of some stuff and now i gave over 15gb of space and I am still having the problem.

Just got rid of more stuff...23gb of space

Have you tired repairing permissions?

how do i repair the permissions?

Open Applications/Utilities/DiskUtility.app
Select the HDD from the left column
Click on "Repair Disk Permissions"
After its completed restart the MB.(Just in case.....)

I am still having the same problems.

Ok I found out what the problem is
http://docs.info.apple.com/article.html?artnum=304807

Ah - well done!

Plus now you have plenty of room again as well!