You may get this message when sending and saving emails to Salesforce if you do not have write permission on the Description (Comments) field in Salesforce. This field is where the body of the email is saved. To check to see if this is the case:
1. Click Setup.
2. Click App Setup > Customize > Activities > Task Fields
3. Click on the Comments or Description Field.
4. Click Set Field-Level Security
5. Check that the field is not set to Read Only for your Profile, and that it is visible.