In some conditions email's are working properly.

1. Email is working on user registration.
2. Reset password etc..

But not working in some cases

1. New order creation
2. Sending emails from the order page in the admin panel.

Is this something related to server, I've tried to log the errors but it is empty.

Server: Ubuntu

Magento Version: 1.9.1.0

Please suggest if someone had the same issue, I can see someone had the same issue in some other community but no one answered for that question.