There are few protections, although the letter to my class mailing list came with a warning from the ISP that it might be false. Anonymity is a very important part of Internet discussions, although not all groups allow it. It is disproportionately used by the discontented, including some students who seem emotionally unbalanced, but there is little harm in giving them an outlet. When they attack me unfairly, other students generally come to my defense. Using a mailing list, which automatically send email to everyone's box, for this purpose is not such a good idea, however.
People quickly complain about getting so much mail from a few malcontents.
They occur both in anonymous groups and groups where people are identified. The Internet attracts many people who otherwise have a hard time finding an audience for their preoccupations. In the case of the Marxist Sociologists, it provides support to people who otherwise find it difficult to get anyone to take their ideology seriously. I am sure this is true for other eccentric groups as well, and it may increase the tendency for people with unusual beliefs to isolate themselves from society at large.