Recently had a case where the Sent Items folder in Outlook (cached mode) would not show any messages after a specific date. Actually, that is not strictly true, the messages would appear in Sent Items briefly after being sent, but would then disappear.
Having established there were no rules that were moving the messages, and then checking that they were still visible online, I removed the OST file to allow it to rebuild, and all the missing messages were downloaded and appeared as you would expect… Right up until the time Outlook showed the folder was up-to-date, at which point the messages (again from the same point in time) all disappeared again!
I opened a case with Microsoft and they had no solutions, but did eventually point me to a suggestion in a forum and this – at the bottom of the first page – did the trick:
Set Outlook to work online (non-cached) and copy all the messages out of the troublesome folder into a temporary location (e.g. an empty “temp” folder). Then delete the contents of the original folder (Sent Items in this case) and then copy the messages back again.
I elaborated on the process a little by turning on cached mode after the initial copy and delete, and waiting for all folders to be up-to-date, before copying the messages back again (the idea being that the online empty Sent Items should sync back to the local copy to ensure it is empty before you then copy the originals back). So far seems to have done the trick!
I suspect there was a hidden and/or corrupt message within the folder, and may well have been able to identify it with MFCMAPI, but the above was a relatively quick fix (once we found it!).