Spoke to MOM officer (MInistry of Manpower) that she noted me Contract of Service is a mutual agreement between employer and employee, even no documentary proof is considered employment agreement, includes email but not SMS. (short message)
In my case, though I never signed any documents, is considered a mutual agreement between Boss and me to allow me to work for my Boss, as employment agreement.
So in future, should ask for Letter of Appointment / Employment in interviews to sign before starting the jobs. If no such document, how can the employees claim salaries if there's no documentary proof and the employers refuse to issue salaries? There were employers refusing to bind mutual agreement as employment agreement if it includes email. They can just send SMS for employees to start work.
Looks like MOM only cares employers to save paperwork and can sue employees off-guarded. Also MOM said employers can sue employees not under Employment Act in Civil Court, as some employees in executive and managerial posts, or so earn more than S$2,500 per month not under the MOM Employment Act.