Dear Eric and Mithun,
Thank you so much for your valuable time and quick response.
I got the cause and solution to this problem is also simple.
Casue : The issue was caused due to the SMTP server of gmail. As per google's policy we cannot send mails in bulk if we are a normal account holder on gmail and we use same constant subject line for bulk mails. If we use, then after certain mails (Don't know exactly how many 
), google will treat it as spam and block the mails you send.That information I got from my ISP.
Solution : I changed the subject line for each mail and kept it variable so that on each mail I will get the different subject lines for different mails and it worked. Now I am able to get the whole mails.
Regards,
Vinod Memane