Hi Sayeed,
As Rishi said, please check with mail team as issue may resist with Lotus Notes mail protocols. For testing purpose , try to send mail from your SAP server to any GMail ID and check. As you confirmed that it shows SUCCESS status in SOST, so for no doubt from SAP side, it delivers mail, but due to some interface issue (like mail server constraint ), it's not getting delivered. If you get mail in GMail, you can confidently ask MAIL team to look into the issue 
.
Regards,
Nilutpal.