In SkyConnect, the validation of the recipient user is done during the first phase of the SMTP mail transaction. In this configuration, bounce / ndr is not sent for non-existing recipients. Instead an error message pop-up is shown to user in email client like Baya / Outlook etc. 
This is more usable for the end user, as an error is immediately shown and it can be corrected and resent. This approach also provides more security against misuse of the mail server. 
In SkyConnect, NDR/bounce is sent when the mail delivery fails after the first phase, due to any reason.

 Eg. bounce can be sent for mail policy rejection or  when mail cannot be delivered to a remote recipient.