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”;
~#