Redmine cant recognize domain/hostname but I can send emails from command line

I am using helpdesk plugin, so I can reply on tickets and send them back to the customer via redmine.

But I just ran into problem, which I can't solve for few days. I have postfix relay server on external server which works for most of my projects smtpinternal.domain.tld

I have this in my redmine config like this

# default configuration options for all environments
default: 
  # Outgoing emails configuration
  # See the examples below and the Rails guide for more configuration options:
  # http://guides.rubyonrails.org/action_mailer_basics.html#action-mailer-configuration
  email_delivery:
    delivery_method: :smtp
    smtp_settings:
      enable_starttls_auto: false
      address: 'smtp.smtpinternal.domain.tld'
      port: 25
      domain: 'smtpinternal.domain.tld'
      tls: false

Sadly, when I want in my redmine instance try to send test email I have this error message

getaddrinfo: Name or service not known

weird thing is that, I wanted to send test email with mail utility in the container, to test if I can resolve hostnames, and it worked. I can even ping on my Nameservers .. any idea, why redmine refuse to send emails?

Start with the Redmine email_delivery.smtp_settings shown in the issue and compare its hostname resolution with the working mail utility inside the container. Check the container's runtime networking and DNS context before tracing the SMTP connection; done means Redmine can send the test email through the configured relay.