To provide a mail service, Plesk supports two mail transfer agents: Postfix and qmail.

Plesk uses only one mail transfer agent at a time. You can check which of them is currently enabled on the following page: Server Administration Panel > Tools & Settings > Services Management. You can also do this by running the mailmng utility located in the $PRODUCT_ROOT_D/admin/sbin/ directory, where the $PRODUCT_ROOT_D is /usr/local/psa for RPM-based systems or /opt/psa on DEB-based systems:

./mailmng --features | grep SMTP_Server

By default, Plesk for Linux uses the Postfix for sending and receiving mail through the SMTP and SMTPS protocols. You can switch to qmail by running the following command:

# /usr/local/psa/admin/sbin/autoinstaller --select-release-current --install-component qmail

To switch to Postfix run the following command:

# /usr/local/psa/admin/sbin/autoinstaller --select-release-current --install-component postfix

Both Postfix and qmail use the same root directory to store incoming mail. This directory is defined by the variable $PLESK_MAILNAMES_D in the /etc/psa/psa.conf configuration file. By default, it is /var/qmail/mailnames. Storing incoming mail in the same directory allows the messages remain available after switching between mail agents.

Unlike incoming mail, the mail queue is lost while switching between the mail agents. Therefore, before switching, we recommend you stop the SMTP service to prevent the acceptance of email and the delivery of all queued mail. To stop the SMTP service, run the following command:

# /usr/local/psa/admin/sbin/mailmng --stop-smtpd

To flush the queue, run the command:

for qmail: # kill -ALRM `pidof qmail-send`

for Postfix: # postqueue -f