Mail in my inbox just disappears after a few weeks, why?

Question

Mail in my inbox stays there for a few weeks and then disappears, only leaving me a few weeks worth of messages - similarly on my .mac webmail. Can't find any preferences to stop this happening.
Any Ideas?

Answer 1

The same behaviour does not happen with the iMac and so
I use this as backup.
Well, it doesn’t happen there because it stores mail locally instead of on the server but, interestingly enough, the iMac is almost certainly the culprit. What are your Preferences > Accounts > Advanced > Remove copy from server settings there?
I never changed the iMac .mac account from POP to .mac as I
would have had to delete the account and create a new one
and would have thus lost everything.
This isn’t entirely correct. For one thing, you don’t have to delete the POP account first. You can set up the account as .Mac without deleting the POP account, transfer your mail from the POP account’s mailboxes to the .Mac account’s mailboxes, and delete the POP account afterwards. Alternatively, you could create custom “On My Mac” mailboxes and move your mail from the POP account’s mailboxes into them before deleting the account.
Before doing anything, however, be sure to quit Mail and make a backup copy of the ~/Library/Mail folder, just in case something goes wrong. You can do this in the Finder by dragging the folder to the Desktop while holding the Option (Alt) key down, for example. This is where all your mail is locally stored.
Note: For those not familiarized with the ~/ notation, it refers to the user's home folder, i.e. ~/Library is the Library folder within the user's home folder.