White Buffalo Ventures is a Texas company that sets up online dating sites. In early 2003, they had the clever idea to make a public information request to the University of Texas for all of their e-mail addresses, which the univerisity duly answered, and then to start spamming all those addresses. The university asked White Buffalo to stop, which they didn't, then started filtering them. White Buffalo sued, on the theory that the filtering was illegal under the First Amendment, since UT is part of the state government, as well as under the CAN SPAM act.
After percolating through the courts, the Fifth Circuit published its decision that UT can indeed filter out the spam. UT acts as an ISP for its students, and CAN SPAM specifically says ISPs are allowed to implement whatever spam filters they want. The court further decided that in view of the Central Hudson case, the most important Supreme Court decision about the first amendment and commercial speech, that UT's filtering doesn't violate the First Amendment.