After I installed postfix and stopped sendmail and started postfix and rebooted, Virtualmin configuration still says : "Mail server Sendmail is installed and configured."
A few things to be aware of:
Your OS probably also thinks Sendmail is still the boss. Use alternatives to switch:
alternatives --config mta
Then you can reconfigure Virtualmin to know you’re using Postfix by editing the Module Configuration option labelled “Mail server to configure”. Just switch it to Postfix and save.