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)

smtpserver/server_test.go

@@ -530,7 +530,14 @@ func TestDelivery(t *testing.T) {
 
 	ts.run(func(client *smtpclient.Client) {
 		mailFrom := "remote@example.org"
-		rcptTo := "mjl@127.0.0.10"
+		rcptTo := "mjl@[127.0.0.10]"
+		err := client.Deliver(ctxbg, mailFrom, rcptTo, int64(len(deliverMessage)), strings.NewReader(deliverMessage), false, false, false)
+		ts.smtpErr(err, &smtpclient.Error{Permanent: true, Code: smtp.C550MailboxUnavail, Secode: smtp.SeAddr1UnknownDestMailbox1})
+	})
+
+	ts.run(func(client *smtpclient.Client) {
+		mailFrom := "remote@example.org"
+		rcptTo := "mjl@[IPv6:::1]"
 		err := client.Deliver(ctxbg, mailFrom, rcptTo, int64(len(deliverMessage)), strings.NewReader(deliverMessage), false, false, false)
 		ts.smtpErr(err, &smtpclient.Error{Permanent: true, Code: smtp.C550MailboxUnavail, Secode: smtp.SeAddr1UnknownDestMailbox1})
 	})