Weird: receiving and sending old emails

Question

I am both receiving and sending old emails - from about a month ago.
Emails that I've previously received are turning up again in my inbox. They are flagged as newly received, but the send date is from about 6 weeks ago. No message appears more than once and they are appearing in the order they were originally sent.
In addition, my contacts are receiving emails from me again, also sent from that same period, about 6 weeks previously.
There's another thing. Often, an email will appear in the Outbox from nowhere. This can be either an email I previously sent, or an email I've previously received. When I delete it, it goes to trash. However, when I go to inbox, or another folder, then BACK to Outbox, another email will appear in its place.
Weird, huh?
I've talked at length with my ISP about this and they've had me rebuild my account, only for the problem to persist.
Any advice would be greatly appreciated. Many thanks!

Answer 1

Some if not all the problems you’ve described are caused by some index corruption (which in turn might be caused by some filesystem corruption) that makes messages to randomly appear in a mailbox different from where they’re supposed to be. If that mailbox happens to be Outbox, Mail thinks the messages are waiting to be sent and dutifully proceeds to try to send them.
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), try this:
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. Move any “IMAP-”, “Mac-”, or “Exchange-” account folders to the Trash. Note that you can do this with IMAP-type accounts because they store mail on the server and Mail can easily re-create them. DON’T trash any “POP-” account folders, as that would cause all the mail stored there to be lost.
5. 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.
6. As a side effect of having removed the IMAP account folders, those accounts may be in an “offline” state now. Do Mailbox > Go Online to bring them back online.
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.