If you are using Visual Studio Tools for Applications to develop your managed code for an InfoPath form, please follow these steps.
Delete the txtEmailAddress field from your manifest.
DO NOT EDIT THE DATA SOURCE THROUGH YOUR GUI OR THE MANIFEST.XSF PROPERTY. This will break your form!!!!
Edit a custom schema file
In a text editor such as Notepad or in an XML editor, create the following schema. Then, save the schema asCustomschema.xsd.
Once completed, re-add your field by dragging and dropping it across from the Data Source.