You can keep your existing email address when you set up a connection with Lightwire, but there may be a few things to consider, depending on who supplies the address to you.
If your email address is provided by one of the free suppliers, such as Gmail, Hotmail, Outlook and Yahoo!, there should be no trouble keeping the address going, and accessing it as normal.
If your email address has been supplied by another internet provider, like Xtra, Farmside, or Orcon etc, you'd need to have a chat with that provider to make arrangements to keep the address. They may charge a small monthly fee to keep the address going.
You might have trouble sending emails with some providers, due to Lightwire's policy of blocking port 25. If you're affected, you may be best to set up a Gmail account, to use their outgoing server.