Deleting V10 in Library, Mail

Delete that Library/Mail/V10 folder and kiss goodbye to all of your emails, email folders, databases, and Mail configuration. This would be a self-inflicted f-bomb.

Instead, review each email with larger attachments, determine if the attachment is a keeper or not, and if the former, then right-click on the attachment and save it to the Finder (or an external drive to conserve space). Then, if the email is expendable, delete that particular email. Unfortunately, one cannot just delete the attachment without deleting the email it is attached too.

One option is to zip the ~/Library/Mail folder, archive the .zip to an external disk, and trash the Mail folder.

If you then miss any old mail, you can just unzip and import it back.