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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.

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.

Host Ip binding failed

I am trying to connect my genymotion emulator to my localhost. After reading an article I got to know that my emulator is running on virtual host so I need to run my application in the same network
So emulator is on ip 192.168.20.101 So I tried to keep my server on 192.168.20.5
I am using dropwizard server. I got an error java.net.BindException: Cannot assign requested address: bind
I used following code to change my yml file
server:
applicationConnectors:
- type: http
bindHost: 192.168.20.5
port: 8090
adminConnectors:
- type: http
bindHost: 192.168.20.5
port: 8091

I am not sure you can bind IP in inside VirtualBox network. What you can do is configure your device to run in bridge mode, like this:
This way the device runs in the same network as your host, so you can easily access localhost or other machines.

Rundeck not sending emails when in Amazon VPC?

I have Rundeck installed on an instance that is on a VPC in Amazon AWS. This isn't the default VPC - it is one I have created with different subnets and security groups. My issue is this: Rundeck runs and acts fine except when sending email. I get this error:
2014-10-01 18:05:42,758 [quartzScheduler_Worker-2] ERROR grails.app.services.rundeck.services.NotificationService - Error sending notification email to email#domain.com for Execution 109: Mail server connection failed; nested exception is javax.mail.MessagingException: 501 5.0.0 HELO requires domain address . Failed messages: javax.mail.MessagingException: 501 5.0.0 HELO requires domain address
This is only when the instance is created in my VPC. It is in a public subnet that accesses the internet directly via an internet gateway so it has no Nat instance between it. ACLs are default for this subnet, and the security group is set to allow all outbound.
The weird thing is this works in the Amazon default VPC, but not in my created one. They are setup the exact same. Only difference is IP range obviously - both have public IPs and the security groups are setup the same. Sendmail can send email, but Java appears to not be able to.
Any ideas on what the issue is? I've tried a few things and I am stumped.

I couldn't get it to work with Amazon SES, but I did get it to work with Gmail smtp by using the Rundeck groovy config located here: https://gist.github.com/gschueler/5707281

Java Mail Exception

I have currently face a problem when I try to send an email using JavaMail API. The exception I get from my application console is :
"javax.mail.MessagingException: 550 Access denied - Invalid HELO name (See RFC2821 4.1.1.1)"
by the way, I have already set my mail.smtp.auth property to true as : props.put("mail.smtp.auth", "true"), but it still fail, does anyone has idea? or face similar problem before ?

A 550 error is often returned by the SMTP server when the sending hostname cannot be inverse resolved to the originating IP address. This allows mail servers a bit of authentication that the sending client is who it says it is. Unfortunately, many test clients - especially systems behind a NAT device - will have originating IP addresses that don't map to any name.
For example, the machine I am typing this on has an unroutable IP address of 192.168.1.103 and my hostname could be so.example.myhouse which works fine because my router pretends that packets from my desk come from (e.g.) 69.59.196.211 which is my WAN address. However, if you use a props.put("mail.from", "me#so.example.myhouse") the SMTP server may try a DNS lookup and obviously fail for my fictional hostname (that is, one that the global DNS doesn't know of).
Even if I used the DNS name which maps to 69.59.196.211 (e.g. stackoverflow.com) the SMTP server may do a reverse DNS lookup to check that 211.196.59.69.in-addr.arpa maps to stackoverflow.com. If that fails, the SMTP server may consider you a spoofer and return a 550.
Finally, your sending client or every host it its IP address block could be blacklisted by the SMTP server for reasons that you have no control over.
Without more context than you probably want to post (names and addresses of the guilty client and server) I can't be sure that it is an SMTP/DNS problem unrelated to Java so you'll have to check those bits yourself. You can skip the Java altogether and telnet smtp-servername 25 and talk to the server yourself. You'll find RFC 2821 helpful should you try.

replace
props.put("mail.smtp.auth", true);
to
props.put("mail.smtp.auth", "true");
and it works :)