Why can't I open my email?

Question

Three days ago, I clicked on the email icon in the dock and got this dialog box.
Mail cannot update your mailboxes 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 eliminated a few items in my home folder that took up space but that does not solve the problem. On a 80 Gig hard drive, I have 64.51 Gigs available.
I can access my email through my ISP but not through the mail application that comes with MAC.
The email I receive through my ISP is the same I receive through Mail in
Mac. I am able to delete unwanted mail through my ISP but not Mac. Is
the mail being deleted in the MAC mail application too, or is it building up
since I can't access it.
G4   Mac OS X (10.4.9)  

Answer 1

Corruption of ~/Library/Mail/Envelope Index is known to cause this bogus error message in Mail, even when there is enough space available on disk — but you must make sure first that there is actually enough space available (a few GB) and that the error message is indeed bogus.
Verify/repair the startup disk (not just permissions), as described here:
The Repair functions of Disk Utility: what's it all about?
After having fixed all the filesystem issues, if any, and making sure that there’s enough space available on the startup disk (a few GB, plus the space needed to make a backup copy of the Mail folder), try this:
1. Quit Mail if it’s running.
2. In the Finder, go to ~/Library/Mail/. Make a backup copy of this folder, just in case something goes wrong, e.g. by dragging it to the Desktop while holding the Option (Alt) key down. This is where all your mail is stored.
3. Locate Envelope Index and move it to the Trash. If you see an Envelope Index-journal file there, delete it as well.
4. Open Mail. It will tell you that your mail needs to be “imported”. Click Continue and Mail will proceed to re-create Envelope Index -- Mail says it’s “importing”, but it just re-creates the index if the mailboxes are already in Mail 2.x format.
Note: For those not familiarized with the ~/ notation, it refers to the user’s home folder, i.e. ~/Library is the Library folder within the user’s home folder.