Problem with sending email. SMTP seems not working and 550 Relay not permitted, ubuntu

Hello, I have setup my ubuntu server with nginx. I have successfully installed ajenti v control panel with mail options. I created mail boxes. I also installed RainLoop webmail client in server and configured it well. It was working fine after installation. Both incoming and outgoing emails are working correctly then. But after a day, it stopped working. when sending email using Rainloop, its showing authentication error after clicking on sent button. But in RainLoop admin, when I am clicking test in configuration, its showing green. I have also configured in Microsoft Outlook. Incoming mail is working correct using IMAP. But out going mails are not working. I also received some email bounce notice from outlook as below:

Your message did not reach some or all of the intended recipients.

Subject: test

Sent: 06-05-2016 19:33

The following recipient(s) cannot be reached:

'Shouvik Mukherjee' on 06-05-2016 19:33

550 relay not permitted

Please help to resolve this issue, I could not found a solutions on that.