Do you know how to use Send Mail from the File Menu?
Will it work with your Email Client application?
If so, you can use Button Setup... to turn the email field into a button that performs the Send Mail script step. You can specify in the Send Mail dialog for this script step (same dialog you get when you select Send Mail from the File menu), that the To email address be taken from this field.
Please note that this will make it impossible to edit an email address on this layout. If that's a concern, put a button for this next to the email address field instead of turning the field into a button.