In current version of Plesk Automation (PA) only Postfix and SmarterMail are supported. It is installed automatically by registering Service Node with 'Mail server' role:
Administration Panel > Infrastructure > Service Nodes > Add New Node > select role 'Mail server' role
Integration with mail server based on APS package. In future it will be possible to build your own APS package for the integration with third-parties mail servers, such as qmail, IceWrap, MailEnable and so on.
Also you may want to use our Documentation in order to create your own APS Connector for External Mail Servers.