after deploying app, following error displays. any suggestion to correct this error

npm WARN config production Use --omit=dev instead.

backend@1.0.0 start
node server.js
Server is running on port 6911
Error: connect ECONNREFUSED ::1:587
at TCPConnectWrap.afterConnect as oncomplete {
errno: -111,
code: 'ESOCKET',
syscall: 'connect',
address: '::1',
port: 587,
command: 'CONN'
}