Mail wants to rebuild mailbox nearly every day

I'm having this exact same issue and I've tried everything I can thing of to solve it with no success.
I've rebuilt the mail database repeatedly, I've removed in many different combinations the plist files and envelope indexes and started over.
I've even reinstalled Mail.
Nothing is working here.
Does anybody have any better ideas?

Have you repaired the drive using Disk Utility?

Well, I haven't tried that but I will now. Although, I wonder how that may be related since I'm having no other issues with the drive.
I'll try anything at this point.

So when you say you had to remove mail completely, how did you accomplish that? There are some many files scattered in the system lib and user lib and elsewhere that I'm not sure I could find all of it to delete it all.

Thanks so much for your help on this. This problem has persisted for a few weeks now and it's driving me mad. You mentioned that you "followed the instructions" to remove mail from your Mac. It would be very helpful if you could conjure up those instructions.
Also, to reinstall mail did you use Pacifist to accomplish that?
I'm not too worried about backing up sent folders and such since I use IMAP for mail, meaning everything is saved on the server. I think I only need to worry about saving the mailboxes that are "On My Mac."
I'll try to deal with this problem this weekend and report back.
Again, thanks for you time and attention.

This just started happening to me this past week--I thought it might be a 10.5.7 thing. It's beginning to happen every day. I will try to run repair permissions and check the disk for problems but figured I would chime in to let you know it's not just you!

-mike

Well, as you no doubt read above I planned to try, as a last resort, completely removing Mail and everything associated with it that I could find and then reinstall.
I did that and that seems to have solved my problem. Also, as mentioned above by another poster who advised me of this tactic, he reimported his mailboxes back into Mail which then listed them under "Imported" is his Mail sidebar. That works fine but then all you have to do is drag the mailbox out of the imported folder and drop it back on to "On My Mac" and they live there just like they had previously. This is likely the only way you're going to get back to normal again. If you have mailboxes that you need to save, drag them out of your user/library/mail directory, to the desktop or somewhere else before you delete everything. That way you can reimport them or just move them back to that directory.
Finally, I can not say enough about using IMAP as opposed to POP mail if your ISP supports it because I basically never lost any messages through this process. They all stayed on the server. The only messages that I potentially could have lost were those saved "On My Mac." But as I said, I backed those mailboxes up and saved them before deleting OSX Mail.
Sorry, for the longwindedness but I am very much relieved.
Good luck!