First of all please make sure that SmarterMail was tried to be detected by Plesk properly. To force detection ,Login into Plesk panel with admin credentials then go to Plesk, Server, Component Management and click on Refresh button to let Plesk run its internal utility to detect/re-detect SmarterMail (or any other component)
After that please go to Mail Server link from the list of components on this page and choose SmarterMail as default Mail server.
If the problem still persists please try the solution below:
Smarter Mail service is configured through Web Server. The following Web servers can be used for Smarter Mail Web service:
1. Smarter Mail Web Service – native web server shipped with Mail Server.
2. Microsoft Internet Information Services – web server, recommended for use by Smarter Mail.