I use the "defer until" option a lot, so there are many unsent messages in my outbox.
I also have things spread out in many subfolders and offline archive files, so I use the search window <Ctrl-E> and "search all outlook" to show all the emails/tasks/etc associated with a particular issue when I need to work on it.
Recently, when I do a search, and an unsent outbox message comes up among the found items, when I use the arrow keys to go past it and it becomes highlighted, Outlook automatically changes the status from pending to unsent (not sure what terminology you would use, but it sets it so it's no longer italicicized and will no longer be sent when the defer time arrives). It didn't do this until about a month ago, and I have tried many things to change back whatever setting got changed.
I don't remember changing anything before this happened, though I did have some issues with a corrupted OST and had to run the repair tool several times and ended up creating a new profile. A headache since outbox messages aren't stored on the server so I had to resend all of them - but this issue exists even for new messages that weren't affected by all that.
Mostly, this isn't a big problem, except if I want to change the defer-to date on a message, if I open it from the search window, make changes and then hit send, because it is still selected in the search window, it closes the email but doesn't "send". If I do the same thing from navigating directly to the outbox, this doesn't happen.
I tried turning of the viewing pane, and removing all the custom fields from the view, or even changing to different views. But it still does it, and I can't find anything in the options screens that seems likely to help. Can anybody recognize what the issue is?