Alert Checker created but didn't receiving mail

Dear Team,

I have created some alert checkers in observium and I am getting alerts in Observium but I didn't get any mails regarding that alert. I have associated three email IDs and also configured the SMTP settings but still I am not getting alert mail. Also I want to configure that when checker is failed then only then I get the email. Email should be delivered when checker is failed, not every time whether successful or failed. 

Please help me to correct my configuration that where I am wrong and solve my issue as soon as possible.

Thank you