You can switch your Mail Transfer Agent (MTA) on Plesk Panel. Either, Qmail to Postfix OR Postfix to Qmail (vice versa).
1.) First, you have to check the current Mail Transfer Agent (MTA) which is installed on your Plesk server using Plesk utility mailmng:
In the command line just type the following command:
#/usr/local/psa/admin/bin/mailmng --features | grep SMTP_Server
$features['SMTP_Server'] = "QMail";
$features['SMTP_Server_package'] = "psa-qmail";
#/usr/local/psa/admin/bin/mailmng --features | grep SMTP_Server
$features['SMTP_Server'] = "Postfix";
$features['SMTP_Server_package'] = "postfix";
2.)Switching MTA to Postfix using the following command:
#/usr/local/psa/admin/sbin/autoinstaller --select-release-current --install-component postfix
3.) Switching MTA to Qmail using the following command:
#/usr/local/psa/admin/sbin/autoinstaller --select-release-current --install-component qmail
That's all!