How can I restore sent mail from a backup? I like to keep sent mail, I file it periodically.  Unfortunately, I must have changed my settings to delete mail older than 1 month.  So now I'm trying to restore mail from a backup.  (Don't ask me why I can't enter Time Machine.  I guess I should ask that as a separate question.  I can back up but not enter Time Machine!  So I'm trying to do this manually).  I can find 
Sent Messages.ewsmbox
Sent Items.ewsmbox
and I can read that they have lots of good stuff in them, but when I copy them into Mailboxes, they don't show up in my Sent folder when I go to Mail.
Have you tried importing those .mbox files?

In the Mail.app menu bar:

File -&gt; Import -&gt; Import Mailboxes...

and point to your .mbox files