Hello
You can use Rules to send the Mail (submit through data conection)on siging specific data in the form.
Open the section properties and check the check box"Allow users to delete the section" to enable the Rules tab
In the rules you can add action based on the Condition(Section is Signed). then enable the submit to email the form.
The other section which is not signed can be hidden using conditional formatting
Kalyan G Reddy
Infopath MVP 2007 and 2008