Usually the missing "Sent Items" folder indicates the folder has not been assigned or created.
This is a simple (3) step process to resolve the issue.
STEP 1) Login to your Smarter Mail Account, Click "Settings", Click "Account Settings", and then Click the "Compose TAB"
STEP 2) Check the "Enable Sent Items Folder" box as shown in the screenshot below.
(be sure to scroll down past this image and complete Step 3 also)
STEP 3) Click the + at the top of the page to add a new folder.
Assign the folder to "root folder" and type "Sent Items", and then hit "SAVE" (as shown below):
