1. Log in webmail by using URLs:
http://www.YourDomain.com/webmail/ or http://www.YourDomain.com:2095
2. Select the Setting Icon and go to Preferences then click on Mail on the top function as below.
3. Select Sent Mail under Compose.
4. At Save attachments in the sent-mail message? Select Save attachments as per below:
5. Click Save to save the setting.