Skip_hosts - blocking traffic to an ESP

KumoMTA Troubleshooting
1

It looks like skip_hosts is, for some reason, blocking traffic to one of the email service providers.

The email service provider rejected some of the emails from us initially because of an RBL listing, but I was expecting KumoMTA to either stop trying (and report the actual bounce message which it did in some cases with a deferral) or try again (if the SMTP code was 4XX).

So from what I can see in the logs, it kept trying for a few hours and then eventually had a permanent bounce with the message:

550 5.4.4 MX consisted solely of hosts on the skip_hosts list

I have no idea why it would update my skip_hosts list as it is hard coded to:

params.skip_hosts = { '::/0' }

I am using traffic shaping with no extra rules, just the community provided rules.

Version: kcli 2025.12.04-9738d291

2

This is how the egress path is created in my KumoMTA configuration.

kumo.on('get_egress_path_config', function(domain, egress_source, site_name)  
  -- Get the shaper's raw parameters without constructing the egress path  
  local params = shaper.get_egress_path_config(domain, egress_source, site_name, true)
     
  params.enable_tls = 'OpportunisticInsecure'  
  params.remember_broken_tls = '10 minutes'  
  params.rset_timeout = '10s'
  params.skip_hosts = { '::/0' }  
  params.ignore_8bit_checks = true  
    
  return kumo.make_egress_path(params)  
end)
3

What’s the destination domain?

4

We’ve seen reports of microsoft hosted domains returning only ipv6 addresses via DNS which can lead to this sort of thing. I’d recommend upgrading to the most recent stable release; see the release notes for the change we made to skip_hosts to accommodate this sort of thing: Release 2026.03.04-bb93ecb1 - KumoMTA Docs

5

The domain is westnet.com.au and is returning both A and AAAA records.

6

Upgraded to the newest version. Will see how we go with it :slight_smile: