"Why Do I Lose Email Messages When I Import Mailboxes from Thunderbird or Postbox to Apple Mail?"
Apple Mail uses a more restrictive standard than Thunderbird or Postbox do when looking for the start of a new message, so it simply misses many or most of them. On one attempted mailbox import, I saw the message count drop about 95%! The scary part is, if I hadn't checked the count in Postbox against the count in Mail, I might not even have realized I'd lost anything. Eudora Mailbox Cleaner, a commonly recommended solution for importing from Thunderbird to Mail, did exactly the same thing, for the exact same message count. (I ran it on Snow Leopard Server in a virtual machine.) So, how do you import without loss? I'm going to make this very simple for you. Use Weird Kid Software's Emailchemy , a brilliant app for converting from one email format to another. In the Preferences, on the "Standard mbox" tab, select "Relaxed" for "Compliance to mbox standard when reading." I also suggest you choose "UNIX and Mac OS X: LF" for &quo