Actually the title doesnâ€™t say it all, because an email client is a program the receives your email like outlook or thunderbird. Thatâ€™s not what you're looking for.
Like jip said unless you want to host your own email you and you don't want to change email addresses all the time you are stuck with free web mail. I think you are freaking out about something that really isn't a big deal. AT&T pays yahoo for their email service, and since you are using AT&T as an ISP you are essentially paying for yahoo email. Their pay email is more secure then their free email. I also have AT&T/Yahoo email that I never use, but when I do check it there is no spam in the inbox. I only get customer related emails from At&T.
Many companies are starting to use services like MSN, Yahoo and Gmail for their corporate email because it's inexpensive, easy, and provides huge inboxes and respectable SPAM controls. I personally use Gmail for Business for my own private email. I get 20 email boxes (only use 2 or 3) I have my own domain name, and the convenience of gmail.
If you want to pay for email an option is to buy a domain name. Most registrars offer either free or inexpensive email packages with your yearly domain subscription. You can even host your own email server. There are several free email servers available.