Although to be fair, it’s entirely possible that the FBI would go over our heads and install this type of equipment on our upstream providers’ equipment. From what I understand, Carnivore sniffs packets on the network, rather than requiring any installation on our machines.
Also, it’s possible that the person to whom you are sending mail or an intermediary network would be monitored. Basically, though, email is a very insecure protocol. You should not assume that any email you send is not being monitored. Sending an email is often compared to sending a postcard; and that’s basically how you should think of it.
If you’re concerned about email security, you should look into pgp, GnuPG or some other method of encrypting your email.
With a well chosen passphrase and some common sense, these programs provide a fairly high level of security (provided no one is monitoring your keystrokes, which is much easier than decrypting messages for the most part).
The only problem with this is that very few people care enough to learn how to use these technologies. This means that you will have very few people with whom to exchange encrypted email.