A very frustrating error which goes something like this.
You type up a bunch of emails and then hit send, they will then sit in the outbox forever not sending. You close down and reopen outlook and the email items have magically disappeared no sign of them in drafts or deleted items.
This issue currently occurs in Outlook 2010 & 2013
Cause: The cause of this issue according to microsoft is the use of Network Archive .PST files , so if you have those attached remove them if you can or move away from them slowly.
The other reason is 3rd party addins, the most common culprit is the itunes/icloud addin.
not so much a solution but a way to recover your emails that were not send.
1. Download MFCMAPI -> http://mfcmapi.codeplex.com/
2. Load the mail profile on your computer if possible , otherwise run mfcmapi on the clients machine
3. Right click the mail profile and click Open Store
4. Navigate to the Recoverable Items folder , Right click and select Open Contents Table
6. Find your email, right click and export message to .msg
there you have it, its not a total loss you can retrieve the emails. It would be nice if Microsoft fixed the issue though
best of luck , hope it helped