Configure IMAP Email account to save Sent messages to it's E-mail accounts Sent Folder.
1. Go to Tools > Account Settings... as shown below.
2. In Account Settings, select the E-mail Account and click Change...
3. In Add New E-mail Account, click More Settings...
4. Now select Folders tab.
5. Choose the 2nd option as shown below.
6. Click the + infront of the E-mail Account to expand it as shown below.
7. Now Click on the + infront of the Inbox of the concerned E-mail Account to expand it as shown below.
8. Click OK. Your Sent folder has been set for the E-mail Account.