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