One of my users has reported the same issue. We have ZCS 7 open source edition as well so this seems to be a common factor. Also this user in particular does use the Web and Outlook interchangeably so this is another common factor. I don't believe this error is reproducible if you use zimbra desktop with IMAP, it will happen if you use HTTP. It doesn't seem to happen with other mail clients. I've tried Thunderbird, windows live mail and outlook express and haven't seen the same issue. I do suspect that if you move around or delete mails in the web and then try outlook it will give this problem, however if you just view mails in the web and in outlook without moving or deleting messages it won't occur (I'll need to test and confirm this). It hasn't become a problem for us as yet because most of our users either use just the web mail or just their preferred client, but it would be good to find a solution to this problem. Hope this information help, for the time being I'll vote for the bug