Mailbox won't rebuild

[J Patrick Draine]

I use Mail on my iMac ( X 10.2.8, 500 MHz), and have accumulated a huge 
quantity of e-mail (like 7K messages) which I read but thought I 
*might* need some day. Now Mail seems to spend far too much of its time 
cataloging  and sorting all that mail (during which the little 
gear-like icon on the right grinds away).
Rebuilding the Mailbox *should* help, but Mail bails out in the 
attempt, indicating something is too corrupted to finish the job (I 
forget the exact wording).
Any thoughts? Should I just print out the really invaluable messages 
and toss the rest & start over again? Putting the unwanted e-mail piece 
by piece is just too labor intensive (as every batch that I delete 
leads to more cataloging etc).
Patrick Draine