Mail app opens hundreds of old emails on startup -- how to fix?

Open your user Library/Saved Application State/ delete the com.apple.mail.savedState file.

If not set to view your user Library, hold down the Option key and select Library from the Go menu in Finder.

Try it without the escaped spaces. After pasting, wait a few seconds to see if a path of folders shows up. It seems to take a while to search for the path. I didn't notice the delay, before.

~/Library/Containers/com.apple.mail/Data/Library/Saved Application State/

You should see a folder named,

com.apple.mail.savedState

Delete that folder.

If it is functional, but slow, try cmd-opt-w.

That should close all windows. It will close the message viewer window, you can open that up from the Window menu.

Clearing the saved state should have stopped those from opening, too.

I don't know why it would open all of those windows.

The outbox.mbox folder would be in an account folder in your user Library/Mail/V9/ folder. (~/Library/Mail/V9)

The account folders have a unique id for name, so you have to just open each and see what is inside. If there is an Outbox.mbox, you could try deleting that. Once you do that, go up one folder and open MailData. Delete all of the Envelop Index files.

Open Mail.

Did you hold down the Option key and select Library from the Go menu in Finder?

The folder is actually an alias to a folder in Mail's container.

Try using Go to Folder (cmd-shift-g) and paste in this path:

~/Library/Containers/com.apple.mail/Data/Library/Saved\ Application\ State/com.apple.mail.savedState

That should open a window to that folder.

I didn't tell you to open a folder and type that path.

I told you to use Go To Folder from the Go Menu in Finder, or use the shortcut (cmd-shift-g).

I guess I could have specifically told you to go to the Go menu, but I figured you would just type the shortcut.