Time Machine fails to restore corrupted POP account sent box

Restore time machine problems

Mailbox corruption / Time Machine problem

My sent box has 5 email accounts. One is broken and not showing any content. I am trying to restore this POP account. 

My filing system is cautious, never letting mail boxes get too large before splitting them. The largest have about 10,000 emails. In the problematic mailbox there are many, many duplicate entries from earlier years, so perhaps corruption has been creeping in for a long time. I estimate there are maximum 15,000 true emails, but Time Machine is showing around 620,000, so no wonder that it’s getting confused. It naturally takes a lot time for TM to find and display them all. I have tried to mark them all to restore and deal with the duplicates later, but TM just locks up. 

It’s 5 years of outbound POP emails, so important, as there are naturally not on any server. Any POP experts here with ideas how this could be solved please?  

(I have a boot clone from April 29, but in using that, I would loose the last days of work entirely). Naturally, the sooner I can solve this problem the better, just in case I need to use the boot clone.