OS X Mail Question

When using OS X mail, I noticed that after changing someone's email address in Contacts, the old email address still shows up in the To: line of the email that I'm sending. Mail will give me a choice of both the old and new email addresses. I don't know where Mail is finding the old email address. I checked my other Contacts and the old address is not listed there either. Is there a way to permanently delete the old email address from OS X Mail?

Thanks,
Tony