Searching around reveals a few articles and such, but most are very old (circa 2007) and not relevant to the newer version(s) of Mail in OS X Lion. The trick to getting the Outbox to stay permanently is to add it to your Favorites bar. But before you do that, you have to be able to see it. Here's how:
- 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.blah
- 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.
- 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.
[crarko adds: I tested this (in 10.7.2), and it works as described.]