installed postfix, stopped sendmail, started postfix, virtualmin still thinks sendmail is the mail server.

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.