Do not use HELLO as fallback of EHLO when server responds with 421

Change inspired by PHPMailer/PHPMailer#2189

RFC 1869 section 4.5 states that the server will return the code
421 if the SMTP server is no longer available

This change fixes an issue where the actual error response from a
failed EHLO was not surfaced due to always being overridden by the
HELLO response.

client.go

@@ -197,12 +197,23 @@ func (c *Client) greet() error {
 
 // hello runs a hello exchange if needed.
 func (c *Client) hello() error {
-	if !c.didHello {
-		c.didHello = true
-		if err := c.greet(); err != nil {
-			c.helloError = err
-		} else if err := c.ehlo(); err != nil {
+	if c.didHello {
+		return c.helloError
+	}
+
+	c.didHello = true
+	if err := c.greet(); err != nil {
+		c.helloError = err
+		return c.helloError
+	}
+
+	if err := c.ehlo(); err != nil {
+		var smtpError *SMTPError
+		if errors.As(err, &smtpError) && (smtpError.Code == 500 || smtpError.Code == 502) {
+			// The server doesn't support EHLO, fallback to HELO
 			c.helloError = c.helo()
+		} else {
+			c.helloError = err
 		}
 	}
 	return c.helloError

client_test.go

@@ -9,6 +9,7 @@ import (
 	"bytes"
 	"crypto/tls"
 	"crypto/x509"
+	"errors"
 	"io"
 	"net"
 	"net/textproto"
@@ -509,6 +510,39 @@ var helloClient = []string{
 	"NOOP\n",
 }
 
+var shuttingDownServerHello = `220 hello world
+421 Service not available, closing transmission channel
+`
+
+func TestHello_421Response(t *testing.T) {
+	server := strings.Join(strings.Split(shuttingDownServerHello, "\n"), "\r\n")
+	client := "EHLO customhost\r\n"
+	var cmdbuf bytes.Buffer
+	bcmdbuf := bufio.NewWriter(&cmdbuf)
+	var fake faker
+	fake.ReadWriter = bufio.NewReadWriter(bufio.NewReader(strings.NewReader(server)), bcmdbuf)
+	c := NewClient(fake)
+	defer c.Close()
+	c.serverName = "fake.host"
+	c.localName = "customhost"
+
+	err := c.Hello("customhost")
+	if err == nil {
+		t.Errorf("Expected Hello to fail")
+	}
+
+	var smtpError *SMTPError
+	if !errors.As(err, &smtpError) || smtpError.Code != 421 || smtpError.Message != "Service not available, closing transmission channel" {
+		t.Errorf("Expected error 421, got %v", err)
+	}
+
+	bcmdbuf.Flush()
+	actualcmds := cmdbuf.String()
+	if client != actualcmds {
+		t.Errorf("Got:\n%s\nExpected:\n%s", actualcmds, client)
+	}
+}
+
 var sendMailServer = `220 hello world
 502 EH?
 250 mx.google.com at your service