Below is a snippet of code you can use to target the invalid email message. This way you can update the text to reflect what you prefer. Simply paste this snippet in the Popup JS tab within the custom code (More info here) and toggle the code "ON".
If you only desire a text to appear with no alert box, take the steps below.
This video shows how you can alter the formatting of the email verification error message within the design canvas.
Tags: Email Error, Error Message, Subscribe Error, Designs, Formatting