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Nginx as reverse proxy mail server host

July 02, 2020 02:20PM

I am trying to proxy a SMTP server on Nginx using the below configuration. I want all the client calls to hit the SMTP server via my proxy host. I want the SSL termination on nginx for the client calls to the SMTP Server.

When I do the connection getting below exception even before the SSL handshake.. Please correct me if I am wrong anywhere.

Without SSL directive & Properties in nginx.conf, it works fine and able to do SSL handshake as well. Not sure how it would be a SSL connection, without the SSL directive and SSL properties.

Java Error
javax.mail.MessagingException: Could not connect to SMTP host: localhost, port: 3001, response: -1
at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:2197)
at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:740)
at javax.mail.Service.connect(Service.java:388)
at javax.mail.Service.connect(Service.java:246)
at javax.mail.Service.connect(Service.java:195)
at javax.mail.Transport.send0(Transport.java:254)
at javax.mail.Transport.send(Transport.java:124)
at com.att.client.smtp.SMTPTestClient.main(SMTPTestClient.java:50)

upstream smtp_backend {
server smtp.gmail.com:587;

server {
listen 3001 ssl;
proxy_pass smtp_backend;

ssl_certificate C:/nginx-selfsigned.crt;
ssl_certificate_key C:/nginx-selfsigned.key;
ssl_protocols SSLv3 TLSv1 TLSv1.1 TLSv1.2;
ssl_ciphers ALL;
#ssl_ciphers HIGH:!aNULL:!MD5;
ssl_session_cache shared:SSL:20m;
ssl_session_timeout 4h;
ssl_handshake_timeout 30s;

Java client code
Properties prop = new Properties();
//prop.put("mail.smtp.host", "smtp.gmail.com");
prop.put("mail.smtp.host", "localhost");
//prop.put("mail.smtp.port", "587");
prop.put("mail.smtp.port", "3001");
prop.put("mail.smtp.auth", "true");
prop.put("mail.smtp.starttls.enable", "true"); //TLS
//prop.put("mail.smtp.starttls.required", "true");

Session session = Session.getInstance(prop,
new javax.mail.Authenticator() {
protected PasswordAuthentication getPasswordAuthentication() {
return new PasswordAuthentication(username, password);

try {

Message message = new MimeMessage(session);
message.setFrom(new InternetAddress("siva.pannier@gmail.com"));
message.setSubject("Testing Gmail TLS from nginx");
message.setText("Dear Mail Crawler,"
+ "\n\n Please do not spam my email!");


Subject Author Posted

Nginx as reverse proxy mail server host

siva.pannier July 02, 2020 02:20PM

Re: Nginx as reverse proxy mail server host

Francis Daly July 02, 2020 05:06PM

Re: Nginx as reverse proxy mail server host

siva.pannier July 03, 2020 08:38AM

Re: Nginx as reverse proxy mail server host

Francis Daly July 04, 2020 04:26AM

Re: Nginx as reverse proxy mail server host

siva.pannier July 06, 2020 12:22AM

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