[quick99si]

Try

1. Messaging / Click Menu, select Options / Message tab on the bottom / ensure that "Keep copies of sent items in Sent folder" is checked

2. Go into the Yahoo account on your phone / Click Menu, select Tools, and then Manage folders / ensure that "Sent Items" is checked. If it isn't, uncheck it, press OK, then go back and enable it again.

Hope it works for you.