What is the 'crossing the line'?

A Belgian newspaper starts off with a headline: Teacher fired for crossing the line of humor.

The Dutch and history teacher, 25, was given an option - be fired or quit - after sending a "racist" email. The email was about a Moroccan Monopoly edition. All the blocks said "to jail", and had "social services" instead of "lucky cards".

The principal is quoted as saying "We have a very clear attitude. We forbid teachers and students from wearing a veil. On the other side, we are even more strict: we do not allow racism. Criticism and humor are ok, and are needed in a democracy. But such an email is base racism." He was disturbed that the email, which was sent to some of the students, was sent only to ethnic Belgians. If it was not racist, he reasons, the mail would have been sent to Moroccan kids as well.

Is such an email really racism? what would be considered critical humor in this case?

Source: De Standaard (Dutch)