Sent box has disappeared in Apple Mail

Check the sidebar in Mail. Look at the exchange account and see if the sent mailbox is not listed there. If so, select the mailbox and in Mail's menu, under Mailboxes, choose Use This Mailbox For and select sent.

Further down in the sidebar, below On My Mac, you should have a folder for the 3 exchange accounts. Expand the folder for that account and see if the Sent Mailbox is listed there.

What happens when you send an email from that account? It should at least create a local folder for that account if the exchange mailbox is missing.

If it happens again, try going into Mail's preferences, Accounts tab, select the account and choose Mailbox behaviors. Uncheck the box to store Sent mail on the server. That will put it back under the exhcange folder.

Then quit and restart Mail, and go back an check to store Sent mail on the server. That may undo the glitch.

You said you had 4 accounts (3 Exc. & 1 POP). Your bottom sidebar shows 2 accounts, but you should have 3. Is the 3rd listed and not shown in the screenshot? If it actually showing 2 accounts, is the 3rd missing from the bottom sidebar the account with the problem?

If the account in question is shown in the bottom sidebar, uncheck store Sent messages on the server. Does the sent mailbox appear back under the Exchange account folder in the sidebar?

Only the IMAP or Exchange accounts will show at the bottom sidebar. When you uncheck to store sent messages on the server, the Sent mailbox should reappear in the Workplace account folder. The fact that it doesn't points to that it's not being picked up from the server by Mail.

Try unchecking it again, save the setting, quit mail and restart mail. Make sure the setting remains unchecked to store sent messages on the server. Restarting mail should have resynced the exchange accounts. Check again to see if the Sent folder appears under the Workplace folder.

If you can't get it to appear under the exchange folder, test this with another exchange account that's working and just recheck the preferences to put it back.

If it does move back under the workplace folder, then sending a message from that account should create a local Sent mailbox for that account under the main sent mailbox. You can send a test message to try it.

The Sent Messages under On My Mac is the local mailbox created when you sent a messages without storing messages on the server. It got pushed out to On My Mac local mailbox section when you rechecked to store messages on the server. You can right click on it and delete it.

The problem is that for that account only, the IMAP sent mailbox seems to be hidden. I don't understand why it won't appear under the exchange folder when you uncheck storing messages on the server. This may be a problem with the account itself. For example, gmail has an option to not show mailboxes as per your choice. But if that were the case, it shouldn't show on other devices.

Let's try these things. Be sure to Quit Mail before doing this.

In the Finder, select Go, then Go to Folder. Paste this in the window:

~/Library/Preferences/com.apple.mail.plist

A finder window will open to that com.apple.mail.plist file. Move it to the trash. This will put your Mail application back to it's default settings. Open Mail and see if the Sent mailbox for that account appears.

If that doesn't work , quit Mail. You can select the com.apple.mail.plist file in the trash, right click on it and select to Put Back if you want your original settings back.

Once again, in the Finder, select Go, Go to Folder and paste this:

~/Library/Mail/V2/

This will open a window to the V2 folder in your Home Library. You will notice a folder for each account. Move only the folder for that specific account to the desktop.

Then open the MailData folder. Select the Envelope Index files and move them to the trash. Open Mail and hit Continue in the message box. This will reindex all messages. This could take a while if you have a lot of messages, so be patient.

Please make sure you have some kind of backup before doing anything. If you don't, then copy the V2 folder to the desktop or some other location so your messages and accounts can be put back if necessary.

This happens to me once in a while as well. It only happens with 1 specific exchange account (I have 2 other exchange accounts which are always fine). I've never figured out a permanent solution, but I delete the entire exchange account from Apple Mail and recreate it. The sent box then magically reappears.

Since it only happens with 1 specific exchange account I'm thinking it has to do with the exchange settings for that particular account and how it interacts with Apple Mail.

Mountain Lion will release this summer so that should hopefully squash this nuissance.

Hopefully it won't happen again. We'll see.