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GCP GKE SMTP port 465 connection reset

I've got a Spring Boot Java application that sends emails via SMTP port 465 (non-gmail email). My configuration:
spring:
mail:
host: ssl0.ovh.net
port: 465
username: mailer#external.pl
password: mailerPassword
Locally everything works fine. When I deploy it on GCP Kubernetes cluster I've got a connection reset exception. I've read that GCP recommends some external paid providers but for my solution it's too much of a complication. Additionally I know that port 25 is disabled on GCP -> I've got port 465.
I've tried to simply add a firewall egress rule to enable traffic on every port for my VPC but it also didn't help.
What am I missing? Can anybody help my solve this puzzle?
Additional info:
My Java application is served as a simple deployment in k8s. It is exposed through LoadBalancer service on port 80.
Making a curl from given managed pod works - I receive a correct 2xx responses from various sites.

It turned out that my config map port changed from 456 to 465 but I forgot to restart the app. It works like a charm without any NAT configuration or firewall rules. Thanks for your support.

unable to send email on docker container from port 465

I tried to send an email from port 465, it works well on IDEA. But when I deployed it on docker, the email send failed. The error message is:
javax.mail.MessagingException: Could not connect to SMTP host: smtpdm.aliyun.com, port: 465, response: -1
Then I tried to change the port to 25, the email sent success.
Change the smtp server seems nothing better.
How can I send mail from port 465 on docker container?

make sure that the port is available on the host machine and ports are mapped correctly when you start the docker image. also, make sure that postfix is installed in your docker image and is active (sudo postfix status or sudo systemctl status postfix).
if both of theses conditions are met but it still don't work please also Try to set these values to:
smtp_tls_wrappermode = yes
smtp_tls_security_level = encrypt
in /etc/postfix/main.cf in the docker container.

Simple Java Mail SMTP host

I am using Simple Java Mail API in order to send emails from my Spring Boot application. The app's owner was my colleague who doesn't work with me now. In my application.properties file I have the following config
simplejavamail:
smtp:
host: 172.28.94.229
port: 25
javaxmail:
The question is that I can't understand what host is this? Which server my emails go to? Is this something provided by Simple Java Mail, but I couldn't find anything in their documentation.

This IP address is not publicly routable on the Internet but is reserved and used for private or local networks.
For you information the following IPv4 address spaces are not Internet routable:
10.0.0.0/8 IP addresses: 10.0.0.0 – 10.255.255.255
172.16.0.0/12 IP addresses: 172.16.0.0 – 172.31.255.255
192.168.0.0/16 IP addresses: 192.168.0.0 – 192.168.255.255
You can try the following command
telnet 172.28.94.229 25
and see if you get a timeout (meaning that no service is listening on that IP/Port) or you are connected to some service.
If this IP/Port exists and you get a connection opened, then you should ask your company's system admin for more information on that SMTP server.
If you cannot get connected, maybe this is just a mock / not real SMTP server.

How to make Spring Boot application accessible by external IP address of the server?

I'm a newbie to Spring Boot. I have a REST API application written in Spring Boot. When I execute my Spring Boot JAR, everything is okay and I can access the REST API with the localhost address instead of the actual one:
http://localhost:8083/articles
But when I try to access the REST API by my external IP address, I can't do it:
http://100.90.80.70:8083/articles
netstat -antu command in the Linux terminal gives me the following output:
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp6 0 0 :::8083 :::* LISTEN
As I understand, my app is accessible only in localhost, because it hasn't a foreign address.
My application.properties file has only this line:
server.port=8083
Also, when I try to add a server.address line to application.properties like that:
server.address=100.90.80.70
server.port=8083
I have the following Exception: Caused by: java.net.BindException: Cannot assign requested address.
So my question is: how to make Spring Boot application accessible by external IP address of the server? Thank you.

As #Mark said, the problem is in the firewall. I have opened 8083 port in the firewall settings and now I can access my REST API app by the external IP address:
http://100.90.80.70:8083/articles
Linux command to check firewall status:
sudo ufw status verbose
Open 8083 port for remote access by TCP protocol:
sudo ufw allow 8083/tcp
More settings here: https://www.cyberciti.biz/faq/how-to-open-firewall-port-on-ubuntu-linux-12-04-14-04-lts/

I fixed the same by configuring port forwarding on my router, to allow traffic from public ip

In my OpenStack environment, after much debugging, the solution was to create a new Security Group Rule, which looks like this:
Security Group Rule.
Note that my Spring Boot application was deployed on port 8080.
I also noticed that on ubuntu18 the firewall is disabled by default. It did not cause any problems.

Unable to connect to Tomcat server with URL

I can connect to my tomcat7 server using IP address but when I replace IP Address with my URL, I'm unable to connect to the host.
Error Message displayed:
This web page is not available
ERR_CONNECTION_REFUSED
The website may be down or your network may not be properly configured.

Try add pair host_ip host_name to host's file in your compruter.