in dns.ParseDomain, don't allow ipv4 addresses (ipv6 addresses were already rejected)

we are expecting a DNS domain name there.
also highlighted a wrong test in the smtp server.

dns/dns.go

@@ -20,6 +20,7 @@ var (
 	errTrailingDot = errors.New("dns name has trailing dot")
 	errUnderscore  = errors.New("domain name with underscore")
 	errIDNA        = errors.New("idna")
+	errIPNotName   = errors.New("ip address while name required")
 )
 
 // Domain is a domain name, with one or more labels, with at least an ASCII
@@ -96,6 +97,12 @@ func ParseDomain(s string) (Domain, error) {
 		return Domain{}, errTrailingDot
 	}
 
+	// IPv4 addresses would be accepted by idna lookups. TLDs cannot be all numerical,
+	// so IP addresses are not valid DNS names.
+	if net.ParseIP(s) != nil {
+		return Domain{}, errIPNotName
+	}
+
 	ascii, err := idna.Lookup.ToASCII(s)
 	if err != nil {
 		return Domain{}, fmt.Errorf("%w: to ascii: %v", errIDNA, err)