Rescue PC

Mac Mail - Get the Outbox back
Go to Mail Preferences and to Accounts.
Pick an account and under 'Outgoing Mail Server (SMTP),' click what's currently selected and choose 'Edit SMTP Server List...'
Create a bogus SMTP server by clicking the +
Description: Bogus SMTP
Server Name: smtp.bogus.com (make sure you select use only this server)
Save the changes if prompted.
Now compose a new test email from this account and send it.
You should see a message indicating the server is offline.
Click the Try Later button.
And now you will see an Outbox in your folder list.
Simply click it and drag it up to your Favorites bar.
It will now stay there forever including after restarting Mail or your Mac.
To clean up afterwards, first go your Outbox and delete the test email sitting in your Outbox
Go back to Mail » Preferences » Accounts
Highlight your account, and change the 'Outgoing Mail Server (SMTP)' to what it was originally.
Next go back into 'Edit SMTP Server List,' highlight the bogus SMTP server and click the - to remove it.

Cheers
Rescue PC