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ip_lookup_strategy

Since: Dev Builds Only

The functionality described in this section requires a dev build of KumoMTA. You can obtain a dev build by following the instructions in the Installation section.

Influences how MX host names are resolved to IP addresses. This value is a string that can have one of the possible values listed below.

The default value is Ipv4AndIpv6 which is how the product behaved prior to the introduction of this option.

  • Ipv4AndIpv6 - Both the A and AAAA records are resolved concurrently, with both being used to produce the effective list of addresses for that MX. This is the default behavior.
  • Ipv4Only - only the A records will be resolved.
  • Ipv6Only - only the AAAA records will be resolved
  • Ipv6ThenIpv4 - resolve the AAAA records first. If none are found or there is an error, resolve instead the A records.
  • Ipv4ThenIpv6 - resolve the A records first. If none are found or there is an error, resolve instead the AAAA records.

If no A or AAAA records can be resolved for any of the MX hosts, messages are considered to be non-deliverable for the associated egress path and the corresponding ready queue will be bulk failed with a TransientFailure with a 451 4.4.4 MX didn't resolve to any hosts response.