Method 1: Completely automatic
If any message in the email conversation thread is already filed, TonicDM will automatically file new replies in the same project. You don't need to take any action for this to happen.
Method 2: CC the Project Email Address
Each project in TonicDM has a Project Email Address. You can file emails you send by CC'ing your Project Email Address.
This will file the sent email in the project record, and it will cause all other incoming and outgoing emails in the same conversation thread to be filed in that project too.
Method 3: Manual filing
You can select the email in your Sent folder and file it using the TonicDM Outlook add-in.