There are two Message Transfer Agents (MTA) supported by Parallels Plesk Panel since version 9, they are Qmail and Postfix. Each time only one of them is used by Parallels Plesk Panel.
MTA that is currently used by Parallels Plesk Panel is shown on the Services management page at Home -> Services Management:

SMTP Server (Postfix)

or

SMTP Server (QMail)

Also it is possible to check MTA using Parallels Plesk Panel utility mailmng:

~# $PRODUCT_ROOT_D/admin/sbin/mailmng –features | grep SMTP_Server
$features['SMTP_Server'] = “Postfix”;
$features['SMTP_Server_package'] = “postfix”;
~#

or

~# $PRODUCT_ROOT_D/admin/sbin/mailmng –features | grep SMTP_Server
$features['SMTP_Server'] = “QMail”;
$features['SMTP_Server_package'] = “psa-qmail”;
~#

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Sometimes we must have to look at the backend files. For example in the case of a mail sending is failed we have to look at the mail log. So it is hard to search everytime for the location of files so I am posting the all Plesk backend files together, Enjoy.

Plesk root directory : /usr/local/psa

Version : /usr/local/psa/version

Admin password is stored : /etc/psa/.psa.shadow

Plesk configuration file : /etc/psa/psa.conf

Restart Plesk : /etc/rc.d/init.d/plesk restart

Apache

Main httpd configuration file : /etc/httpd/conf/httpd.conf

Plesk httpd : /etc/httpd/conf.d/zz010_psa_httpd.conf
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