Cathy has a large client base and has been using Yahoo mail for her business for years. Leo says that Cathy should have her own domain (i.e. Cathy@[insertnamehere].com) and have Yahoo forward to her email service. As long as it forwards, everyone has the same email and she can change her email provider on the fly. Leo also thinks that Cathy should leave Yahoo and go with Gmail instead. Her business is too important to trust Yahoo with it. Gmail also uses IMAP, which allows her to keep copies of the email both on the server and on her PC. Forwarding is easy on GMail also. In fact, check out Google Apps for Business. It's dirt cheap.
Leo also advises to avoid being called a spammer by signing up with MailChimp. She can mail via location, interests, and more. What about third party email clients? Leo has used FastMail and IslandEMail, but Leo says Gmail is the best.