Identifying himself as fictional characters such as "Lord Voldemort", a disgruntled insurance agent sent anonymous threatening messages to some clients after they cancelled policies they had bought from him.
Myanmar national Ye Lin Myint, 36, who had worked for Prudential, was also unhappy as others had failed to turn up for their scheduled appointments.
Yesterday, District Judge Marvin Bay said the Singapore permanent resident's offences showed a "remarkable degree of planning" before sentencing him to two years and five months' jail.
Ye Lin Myint pleaded guilty to five counts of criminal intimidation and eight harassment charges. Thirty other charges for similar offences were considered during sentencing. The court heard that he targeted 33 people in all, including his former clients and their neighbours.
He sent them threatening letters and e-mails in August and September 2017.