Hi Damien,
Fernando is right, you can use JavaScript to do this, but you will need to do the following:
In the Single Line Text box for email, go the control settings > Advanced >Store Client ID in JavaScript variable > Set to 'Yes'.
In Client ID JavaScript variable name, set the value to 'varControl'

Save the Settings and then open 'Form Settings' in the designer ribbon. Expand Custom JavaScript and paste the code from Fernando in his post.
NWF$(document).ready(function(){
NWF$('#' + varControl).change(function(){
this.value = this.value.toUpperCase();
});
});
Save your settings, and then test it out. You'll see the value change when you lose focus on the Single Line Text box.
Thank you,
Sean Fiene