Hi Arun, even with the SMTP options installed, SDM will still attempt to use its own send-mail/huneymail server to send out emails for notifications. You would have to look at those emails you are getting and try to figure out which notification and notification rule they are coming from. Also you can go into Service Desk and from the Service Desk tab, click on View > Notification History, then click search, and it will show you all of the notifications that went out. You can click on any notification, and then click on "View Context" and it will take you to the ticket or document for which that notification was sent out. You can then take a look at the ticket activity to see what triggered the notification and try to reverse engineer it that way.
If what you want to do is completely disable notifications, you can simply run a sql query to blank out all email addresses in the ca_contact table, OR you can rename the pdm_mail_nxd.exe executible in the bin directory and then recycle SDM and the pdm mail daemon wont start, so it wont send out any notifications, however the mail queue may fill up and need to be emptied once in a while to prevent it from taking up a ton of space.
Hope this helps,
Jon I.