Email messed up ever since upgrading to Yosemite. The mail icon remains on the toolbar and I can open if I click into full screen and read the emails. I cannot send any emails. This is a huge problem.

Thanks to all of you for your help.

I am afraid I'm not able to implement any of your recommendations, since I'm not skilled enough.

I therefore will go to an Apple Store ( in Paris) to ask if someone in the shop can implement the necessary recommended actions.

Thanks again

I followed the instruction provided above and nothing happened to change the Inbox which is missing hundreds of emails. Other folders are intact.

Am in my second attempt to re-index mailboxes (running El Capitan). Each time I get 90% of the way through the downloading message process (5+ hours) and it stops. What do I do to get it going again?

I moved the file, only found index file and reopened mail. It did not prompt me and opened but still with two iCloud mail inboxes and neither will allow me to end from them

Mail (El Capitan): Mail troubleshooting

Mail (El Capitan): If you can’t send messages

Mail not sending or receiving messages after updating OS X

Did what you mentioned but after mail app. rebooted some mail came back. But was gone as soon as I selected one, read it, then it disappeared from the mail list - just as before.

thanks anyway.

All I get in the MailData window is the Accounts plist folder, and moving that to the desktop and relaunching Mail does nothing to recoved all my lost emails. Advice?

I give up -- none of the "solutions" works for me. Luckily, all 20,000 ( ! ) of my lost emails in Mail were copied into my Gmail account. All's well that ends well.