kumomta/docs/reference/kumo/make_egress_path/prohibited_hosts.md

prohibited_hosts

A CIDR list of hosts that should be considered "poisonous", for example, because they might cause a mail loop.

When resolving the hosts for the destination MX, if any of the hosts are present in the prohibited_hosts list then the ready queue will be immediately failed with a 550 5.4.4 status.

The default value for this is:

• 127.0.0.0/8, the set of IPv4 loopback addresses
• ::1, the IPv6 loopback address

{{since('2025.12.02-67ee9e96')}}

The default value is now:

• 127.0.0.0/8, the set of IPv4 loopback addresses
• 0.0.0.0, the IPv4 Any address
• ::1, the IPv6 loopback address
• ::, the IPv6 Any address