Old mail messages remain unchanged and are available. However, the mail queue is reconfigured while switching the MTA, and any messages in the queue are lost. Before switching, it is recommended you wait until all queued mail is delivered and stop the SMTP service to stop receiving emails.
The location of the directory where email messages are stored (/var/qmail/mailnames) and the mail format are not changed when the MTA is switched.
To flush the queue, use the following command:
kill -ALRM `pidof qmail-send`
You can stop the SMTP service using the following command:
To switch the MTA from Qmail to Postfix, use the following command:
plesk installer --select-release-current --install-component postfix