One thing that bugged me that I finally figure out: how to drag and drop to file messages. There's a blank column on the left of the messages. If you click and drag next to the message, a message icon appears which you can drag to one of your self-created folders. It's a time saver. Presumably...
Q: Why is there no option to retrieve messages that have already been read??
A: There is. If you want to leave the messages on the server to be downloaded again to another computer:
Go to PREFERENCES. Select ACCOUNTS. Highlight one of your accounts. Click EDIT. Click the ACCOUNT OPTIONS...