diff --git a/client.go b/client.go
index 66249b5..aaea16b 100644
--- a/client.go
+++ b/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
diff --git a/client_test.go b/client_test.go
index 52aea84..296f49f 100644
--- a/client_test.go
+++ b/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