How do I unlock my Juno mailbox?

My Juno program was corrupted and the mail problems were that I couldn't access my sent file and when I sent out an e-mail it would go into the "out" box and eventually would be sent again (sometimes multiple times).

I'm been wrestling around with an e-mail problem for days. My Juno
program was corrupted and the mail problems were that I couldn't access
my sent file and when I sent out an e-mail it would go into the "out"
box and eventually would be sent again (sometimes multiple times). I
have reinstalled Juno but still no sent file. This pops up
occasionally on the screen:

The mailbox "-/Library/Mail/POP-mymail@mail.juno.com/Sent
Messsages.mbox is currently locked by "My-Name-computer.local".
If you open a mailbox that is already in use you may damage it.

I cannot figure out what to do and can't seem to find anyone who CAN
tell me what to do. It is probably something very simple, and I'm
hoping that is the key to the problem.