`When I try to send email I always get 502 error. The same code works nice on my local machine.

here is my log from /var/log/nginx/error.log

2017/08/15 05:45:34 [error] 1355#1355: *1187 upstream prematurely closed connection while reading response header from upstream, client: 188.227.127.215, server: 128.199.206.12, request: "POST /register HTTP/1.1", upstream: "http://127.0.0.1:8000/register", host: ...., referrer: .....

what I'am doing wrong?