Sent emails disappeared after rebuild

Question

When I try to view sent mail from one of my accounts in Mail, I've been seeing "The message xxx concerning “xxx” has not been downloaded from the server. You need to take this account online in order to download it." Checked the size of the box and it was at 1.1 GB, so I deleted a bunch of emails with attachments, but that didn't help after re-opening the application. Following the advice of a post here, I tried to rebuild that mailbox, and the emails all vanished. Tried finding them in Home/Library/Mail/SentMessages.mbox, but the sent mail files there are very small (20 KB), nowhere near the 400MB I was at when the emails disappeared.
I also discovered that there are two, near-duplicate folders for this account in the Mail folder: one contains a trailing space after the name; dates created 3/25/2005 and 1/18/2006, both last modified today. I'm using POP on OS 10.3.9 and have tried re-importing the mailbox, but got nothing. I still get the message about downloading the message from he server when I try to view Sent emails from this account....
Can anybody help me get those sent emails back in their folder? Thanks in advance--

Answer 1

What appears below is what I would have posted if it wasn’t because of the weird duplicate account folder you mention. You may be able to figure out which of the two account folders is Mail using to store your mail by looking at the modification dates of the mailboxes it contains rather than the modification dates of the account folders themselves...
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It looks like you have the problem described in the following article:
Overstuffed mailbox is unexpectedly empty
As you can see, letting a mailbox grow indefinitely is a really bad idea — and it appears to be too late now to solve the problem doing what the article suggests. If this is a POP account, you may be able to fix it as follows:
1. Quit Mail if it’s running.
2. In the Finder, go to ~/Library/Mail/POP-username@mailserver/.
3. Locate Sent Messages.mbox and move it to the Desktop.
4. Open Mail. A new empty Sent Messages.mbox will automatically be created within the account folder, and this will allow you to continue using Mail normally while trying to solve the problem.
Although Sent Messages.mbox appears to be a file, it’s actually a special kind of folder (a package) that contains several files. Ctrl-click on Sent Messages.mbox and choose Show Package Contents from the contextual menu to see the files it contains. Of these files, mbox is the most important and is where all your messages are stored. What’s the size of that file? Depending on its size, you may or may not be able to directly import it back into Mail in the next step.
An Incoming_Mail file may also be present, in which case it might contain messages that Mail couldn’t transfer to mbox. Incoming_Mail is also a standard mbox file like mbox proper, just named differently, and can be imported back into Mail in the same way.
5. In Mail, do File > Import Mailboxes and follow the instructions to import the Sent Messages.mbox that’s on the Desktop. I’m not sure what the import options available in Mail 1.x are, but you should choose Other / Standard mbox or something like that, so that Mail looks at the mbox file only (and Incoming_Mail, if present) and ignores the other files in the package.
If Mail doesn’t let you select Sent Messages.mbox in step 5 because it’s a package instead of a plain folder, rename Sent Messages.mbox to just Sent Messages (i.e. remove the .mbox suffix) so that it becomes a normal folder, and try again. You cannot do this directly in the Finder because removing a suffix by normal means causes the Finder to hide the suffix instead of renaming the file. To remove the suffix from the name, you must do File > Get Info (⌘I) on the file and change the name there.
If the mbox file is too big for Mail to handle it, however, you’ll have to break it into smaller pieces. You can do that with an MBOX Breaker Script available at Allan Sampson’s website, or with Emailchemy. The former will ignore the Incoming_Mail file that I mentioned previously, but you may try importing that file directly if it’s not too big or just ignore it if all your messages are in mbox. Also, if you removed the .mbox suffix from the name of the mailbox, you may need to add it back again for the script to recognize it.
If all is well and you don’t miss anything, the files on the Desktop can be deleted, although you may want to keep them for a while, just in case.
Note: For those not familiarized with the ~/ notation, it refers to the user’s home folder. That is, ~/Library is the Library folder within the user’s home folder, i.e. /Users/username/Library.