"Sent" folder showing incorrect addresses in "To:" field

Question

I'm using Apple Mail on a Macbook Pro. It was working just fine for months, but in the last couple of days I've been having a problem with my "Sent" folder. I have four accounts, each with their own folder in the Inbox and each with their own Sent folder. All of them are pop accounts. The problem I'm experiencing seems to be confined to a single Sent subfolder, which corresponds to a Gmail account.
Apple Mail seems to be scrambling the outgoing addresses that appear in the "To:" field in the list of sent mail. For example a message that I sent to "[email protected]" might show up in the sent mail list as being sent to "[email protected]", which is an address where I used to receive mail! When I click to read the message, the "To:" field on the message itself (and all the headers) appears correct. It's only in the list that there is a problem.
I tried all the obvious things: restarting the program, restarting the computer, and finally rebuilding the sent mail folder. Prior to rebuilding the mail folder it was only a couple of messages sent out in the last couple of days that were affected. After rebuilding, the problem seems to be affecting the entire folder. Every message has an incorrect address listed as its receiver. I rebuilt it again just to see what would happen, and it seemed to fix itself for a moment. However as soon as I clicked to another folder and clicked back, the list was screwed up again.
This is extremely frustrating... does anyone have any info or insight?

Answer 1

Verify/repair the startup disk (not just permissions), as described here:
The Repair functions of Disk Utility: what's it all about?
After having fixed all the filesystem issues, if any, and ensuring that there’s enough space available on the startup disk (a few GB, plus the space needed to make a backup copy of the Mail folder), proceed as follows to re-create the whole index:
1. Quit Mail if it’s running.
2. In the Finder, go to ~/Library/Mail/. Make a backup copy of this folder, just in case something goes wrong, e.g. by dragging it to the Desktop while holding the Option (Alt) key down. This is where all your mail is locally stored.
3. Locate Envelope Index and move it to the Trash. If you see any other “Envelope Index”-named file there, delete it as well.
4. Open Mail. It will tell you that your mail needs to be “imported”. Click Continue and Mail will proceed to re-create Envelope Index — Mail says it’s “importing”, but it just re-creates the index if the mailboxes are already in Mail 2.x format.
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.