UW's policy on the use of college resources for political activity is quite clear, and can be viewed at http://www.wisconsin.edu/govrel/camprule.htm
That page includes the following sentence: "You may not use state resources to engage in political campaign activities at any time"
A few paragraphs later the page states that "state resources" includes university email accounts.
So, at the very least, the "professor" violated university policy. If your assertion is correct (that he's not an "official"), then he can make that argument in court.