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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: Version 2025.12.02-67ee9e96

The functionality described in this section requires version 2025.12.02-67ee9e96 of KumoMTA, or a more recent version.

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