Anonymous complaints should be classified as 2 types:
Complaint with specific details on the incidents, names of the persons involved, time and place
Complaints with no detailed content
If the first type of anonymous complaint is received, the IC may take it up as part of Prevention measures along with HR’s support to create a safe work environment. The issue can be investigated and persons involved are to be interviewed. The issue can be concluded based on the evidence received. Though this will not be treated like a case triggered by a formal written complaint from a victim.
However, not much can be done with a vague email that has no inputs for an IC to begin their investigation.