Stalking involves unwanted contact. In most cases it involves a combination of sending letters, e-mails or messages, making phone calls, visiting home, school or work. It may be a stranger, or someone you have forgotten for a long time, but it usually involves an ex-partner.

Article 285b of the Dutch Criminal Law:

A person who unlawfully and systematically intentionally invades another person’s privacy with the intention of forcing that other person to do something, refrain from doing something, or tolerate doing something, or to instigate fear, shall be liable to a term of imprisonment not exceeding three years or a fourth-category fine, as guilty of harassment.

So this is the point:
systematic invasion of a person’s privacy.

The Dutch Supreme Court takes into account the nature, duration, frequency and intensity of the conduct, the circumstances under which it took place and its influence on the personal life and freedom of the victim.

Systematicity must be such as to give the conduct an intrusive character, it being understood that the effect of the conduct on the victim is assessed according to objective criteria.

It is not always required that the infringement is significant and does not always have to show serious emotional consequences, a major disruption of daily life or a very serious or profound impact on the victim’s personal life and liberty.

Frequent repetition of behaviour that individually constitutes a minor invasion of a person’s privacy can therefore be seen together as stalking.
In most cases, the judge does not make high demands on the systematic nature of stalking. Certainly if the obnoxious conduct is accompanied by threats or insults, it is quickly regarded as intrusive systematic harassment.