Hey Vikrant!
Thanks for staying patient! I had another look and I think that I determined the issue. You have a mail which mandrill rejects sending to. I improved the error reporting and now one is able to see the email that has been rejected & what the reason for this rejection is.
In your case after the user registers the system tries to send a mail to your administrators. But one of those mails soft-bounces.
Emailing to pra****esh@ar***ize.com was rejected by Mandrill for the following reason: soft-bounce. Trying to send the mail using phpMailer.
Here is what the mandrill docs say about bounces:
help.mandrill.com/entries/21751262-What-...if-an-email-bounces-
Now another thing that wonders me is - why is this thing shown to the user. It should be actually logged to a file. I'll need to look into this - most probably there is some setting that needs to be turned on.
Other than that - the registration emails should be sent to the user. Mandrill only rejects the above email, but it sends to every other.
Regards,
Daniel