I read a posting on how to speed up Mail.app (my email client) which involved deleting the Envelope file and allowing Mail.app to re-import all emails and rebuild the file.
On doing this I got the dialog box above – 108,741 emails? Wow! I know I have around 10 years email stored but I never realised it would come to over 100,000 emails!
I’ll update this post with the results of the rebuild (in about an hour!).