A corrupted email crashes Mail

Hi,

• MacBook Pro 15" mid-2012
• El Capitan 10.11.6

This is my wife's computer only used for mail and web browsing.

Mail works with a POP server and once a message is downloaded, it is automatically deleted in the server.

There are about 17 unread messages since the last time Mail didn't crash.

If I open Mail while keeping the Shift key depressed, no crash and I can access the various mailboxes (Smart, Junk, Trash, Sent, ...) but as soon as I try to click on the In box, Mail crashes.

What I have done so far:

• checked the HDD (both with Onyx and Disk Utility)
• rebuild the Mail box
• reindexed Mail by deleting "envelope.*.*)
• complete clean up with Onyx
• tried to open Mail being disconnected from internet but no luck

I suspect there is an unread corrupted email in the In box but as the emails are immediately deleted in the POP server once they have been downloaded, there is no way to get rid of them.

The only way, I think, would be via the Finder to access the Library and the folder where the last messages are stored and delete them there.

Problem: I haven't got a clue which folder to look for ?

Any help would be appreciated.

Michel