Mail Message Not Downloaded Error

Apple's Mail program keeps each inbox or folder in a separate MBOX format file on your hard drive. If you are one of those people who leaves everything in your inbox (as opposed to filing it or deleting it), you will eventually begin to get the following error:

The message 'xxx' has not been downloaded from the server. You need to take this account online in order to download it.

What is going on is that your inbox has gotten larger than the RAM available to Mail for handling it. You must quit Mail, re-open it, then proceed to delete as much email as you can bear, and file into other folders as much email as you can.

Regular filing of email into other folders, and regular clean-out of your sent mail (file or delete these as needed), and trash, will give you a faster, more responsive, Mail program.