Chris Nolan at Trinity University wrote the following in an email message on October 7, 1998:
I was told by my book editor today that L-Soft, the owner of the Listserv software, sent a letter to the American Library Association about the use of the term "listserv" in some recent ALA publications. L-Soft claims that the term is trademarked and therefore cannot be used as a generic term for these sorts of bulletin board/mailing list systems, much like Xerox not being used as a generic for photocopying. Although I had only used the term once in my manuscript, ALAs editors felt that I should either capitalize the term to refer to the L-Soft software or use other terms to describe the more general concept.
Checking L-Soft Internationals web site, I see that they clearly state that LISTSERV is a trademark of their company.