I want to complicate chuck’s answer a bit. Unlike other written words, e-mail is sent out over lines that are owned and operated by a private company. As such, if they were so inclined, I believe a service provider could terminate your service on any grounds it wishes, including offensive speech in an e-mail. Now that would mean that the company is monitoring your electronic mail, which they say they don’t do (and we have no reason not to believe them). But for the sake of argument, I don’t believe there would be anything strictly illegal about an ISP reading what you write and then refusing you service because of it.
Does that circumstance constitute an infringement on your right to free speech? I’m not sure. A bit, maybe, especially if, as is the case in some areas, there aren’t many ISPs to choose from.