Thank you for posting here in the Microsoft Account forums and we apologize for the delayed response.
I understand that you had stopped seeing sent items on your sent folder since 02/24/2014 in Windows Live Mail [WLM].
Initially, please sign in to Outlook.com or **mail.live.com** via web browser. Send a test mail to yourself afterwards and see if it will be registered on sent folder or if an error would appear.
Regarding WLM, please remove and reconfigure the account. You may view the article 'Set up an email app with Outlook.com'
Doing the reconfiguration may fix this. We look forward to your update.
Regards,
Roland