Configure IMAP Email account to save Sent messages to it's E-mail accounts Sent Folder
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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.