Email addresses are just like any other customer contact information e.g. Phone numbers, addresses, etc. They can change and need to be updated.  

Some reasons that email addresses no longer work are

  • Customer has changed email providers
  • Anti-virus software is blocking the receipt of the email
  • Customer has flagged your email as spam
  • Choose not to read the email and simply delete it
  • Etc.

If you are notified of blocked  or undelivered email.  The best way to verify an email address is to send an email to the customer and have them respond to it immediately. That way you know it is working.