Step 2: Select your IMAP account settings via File-> Account Settings-> Account Settings…-> double click on the IMAP account.
Step 3: Press the button: More Setting.
Step 4: Select the Sent Items tab.
Step 5: Enable the option “Save sent items in the following folder on the server”.
Step 6: Select the folder where you want to store the Sent Items in.
Step 7: Click Ok button.