I was looking for an e-mail that I sent so I checked my 'Sent' box in Mail.app and all the messages had the following:
The message from ME <ME@xxx.com> concerning "Reply to Jackie" has not been downloaded from the server. You need to take this account online in order to download it.
The problem is that I'm never Offline. I don't understand what happened to my copies of all these e-mails. Do I have some weird setting? Has anybody else run into this? Is there a fix? Is it a Mail.app problem or my Mail server problem?