I would suggest creating a custom field class, that inherits from TextField and in the display mode creates the mailto tag.
public class EmailToTextField: Microsoft.SharePoint.WebControls.TextField
{
public override void RenderControl(System.Web.UI.HtmlTextWriter writer)
{
switch (ControlMode)
{
case Microsoft.SharePoint.WebControls.SPControlMode.Display:
writer.Write("<a href='mailto:" + Value + "?subject=" + Value + "&body=sometext'>EMAIL</a>");
break;
default:
base.RenderControl(writer);
break;
}
}
}
And then just add it as a safe control, and use it in code like this:
<MyWebControls:EmailToTextField FieldName="Title" runat="server"></MyWebControls:EmailToTextField>
Hope it helps