go/doc/next/6-stdlib/99-minor/net/56025.md

[LookupMX] and [*Resolver.LookupMX] now return DNS names that look like valid IP address, as well as valid domain names. Previously if a name server returned an IP address as a DNS name, LookupMX would discard it, as required by the RFCs. However, name servers in practice do sometimes return IP addresses.