This is happening to over a dozen users now. The emails send, but they don't move to Sent Items, they sit in the Outbox. The user can't tell if they sent for sure until / unless they get a reply. If I turn off Cached Mode the items disappear from the Outbox and if I turn it back on they stay gone. But a day or two later the problem comes back. At the moment I have the 'trouble' users running in non-cached mode, but I would like to get everyone back to cached. Today another user came to me with this same thing, so it appears to be a growing issue. Any ideas on where this is coming from and how to fix it?
I have tried starting Outlook in Safe Mode and deleting the OST file. The AV software on the PCs has no email component. This is happening to people with different size mailboxes, but most of them are fairly small - under 1 gig.