[Koha] Not able to send email alerts

Dear Friends We are using koha for our library (version 16.11) on ubuntu 14.04 I am using postfix for sending email alerts to our user previously it was work very well but now the email alerts not able to send. I have check all settings but not able to solve the query The notices showing pending status in patron detail Kindly check the Screenshot -- Sahadeo D. Taware Librarian, M.L.I.Sc., NET Thakur College of Science and Commerce Thakur Village, Kandivli(E), Mumbai 400101 Mob. No 9324912816 Office 28870627 Extn. 126