Step 1: Open Mail and choose the “Mail” menu in the upper left. Select Preferences.
Step 2: Choose the “Accounts” Tab at the top, then select the account you wish to adjust in the list on the left
Step 3: Select the “Mailbox Behaviours” tab on the right
Step 4: “Store sent messages on the server” should be checked. If it is not, check it.
Step 5: Close the Preferences window and choose to save your changes if prompted
Step 6: Make sure you have selected the “Sent” mailbox on the server as the correct place to store Sent message