Since the change to BT Email a couple of weeks ago, messages sent using Outlook do not appear in my Outlook sent folder, although they do appear in BT Email when I log directly into it on the web. I have updated all the server settings for the new BT Email on Outlook and I have ticked the box in the "save copies of messages in the Sent Items folder". The messages are not disappearing, as some people have experienced, they simply never appear at all. I remember experiencing the same problem a good while back using a previous older laptop with Windows 7 when BT Email was still with Yahoo and there was something about unchecking a different box but I can't find the item on the internet any longer.
Update: I sent an additional message but then realised I could edit the original one - emails sent using my mobile phone or BT Email on the web do appear in the Outlook sent folder. Only emails sent using Outlook don't appear.