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How do I skip IPv6 MX hosts for outbound SMTP?

Depending on the version of KumoMTA, you have two options:

ip_lookup_strategy

You can use the ip_lookup_strategy option to prevent resolving IPv6 addresses for MX hosts.

You can set this as the default in your shaping.toml if you wish:

["default"]
ip_lookup_strategy = "Ipv4Only"

skip_hosts

You can use the skip_hosts option to skip all IPv6 hosts by using the IPv6 CIDR ::/0 which matches all possible IPv6 addresses.

You can set this as the default in your shaping.toml if you wish:

["default"]
skip_hosts = ["::/0"]