My understanding is if a state's board of accountancy can ascertain and verify that you are a CPA, you can put the letters next to your name, when working in that state. Some state do have instant online verification databases.
I'm not really sure what you mean by "certified" or "licensed," as it sounds like something that each state defines differently. For instance, I had the exam parts passed for the a few years and I was trying to complete the experience requirements; I couldn't put the 3 letters next to my name and I couldn't hold myself out to be a CPA, as I wasn't certified or licensed, I was just someone who passed 4 exam parts.
I've heard some accounting firms, sometimes the Big4 or next tier, discourage some from putting the designation in email signatures b/c business transactions that take place across state lines, where some states require you to be certified in their own state before conducting CPA-related activities.
I really can't pass again!