Trusted network and cluster LAN

[hbi99]

Hi there,
Using this helm chart, I have docker-mailserver up and running. When running the following command (from the world) I can reach and "exchange" communications with the server, through the configured load balancer.

nmap -p 25,143,465,587,993 mail.domain.com
---output
PORT    STATE SERVICE
25/tcp  open  smtp
143/tcp open  imap
465/tcp open  smtps
587/tcp open  submission
993/tcp open  imaps

The problem is that a sibling pod/container from the cluster can not reach the mail server the same way. When running the same command in the cluster/container I get "filtered" state for the ports.

nmap -p 25,143,465,587,993 mail.domain.com
---output
PORT    STATE    SERVICE
25/tcp  filtered smtp
143/tcp filtered imap
465/tcp filtered smtps
587/tcp filtered submission
993/tcp filtered imaps

Is this somehow blocked by the firewall because the sibling pod has the IP 10.2.0.152 - and thereby belongs to untrusted network?
Any insights and solution regarding this issue is highly welcome.

[cfis]

A sibling pod/container won't know what mail.domain.com is. If this is a container in the same pod, use localhost. If this is a different pod, use the Kubernetes service name as the host.