Add test for invalid HELO handling in SMTP client

Introduce a new test case to ensure the SMTP client fails gracefully when an invalid HELO command is used. This includes validating error handling and maintaining the client connection integrity. Also, optimize EHLO/HELO command handling by enhancing syntax checking and error response generation.

client_test.go

@@ -1664,6 +1664,31 @@ func TestClient_DialWithContext(t *testing.T) {
 			t.Errorf("client with invalid host should not have a smtp client")
 		}
 	})
+	t.Run("fail on invalid HELO", func(t *testing.T) {
+		ctxDial, cancelDial := context.WithTimeout(ctx, time.Millisecond*500)
+		t.Cleanup(cancelDial)
+
+		client, err := NewClient(DefaultHost, WithPort(serverPort), WithTLSPolicy(NoTLS))
+		if err != nil {
+			t.Fatalf("failed to create new client: %s", err)
+		}
+		client.helo = ""
+
+		if err = client.DialWithContext(ctxDial); err == nil {
+			t.Errorf("client with invalid HELO should fail")
+		}
+		t.Cleanup(func() {
+			if err := client.Close(); err != nil {
+				t.Errorf("failed to close the client: %s", err)
+			}
+		})
+		if client.smtpClient == nil {
+			t.Errorf("client with invalid HELO should still have a smtp client, got nil")
+		}
+		if !client.smtpClient.HasConnection() {
+			t.Errorf("client with invalid HELO should still have a smtp client connection, got nil")
+		}
+	})
 	t.Run("fail on base port and fallback", func(t *testing.T) {
 		ctxDial, cancelDial := context.WithTimeout(ctx, time.Millisecond*500)
 		t.Cleanup(cancelDial)
@@ -1760,20 +1785,6 @@ func TestClient_DialWithContext(t *testing.T) {
 
 
 
-// TestClient_DialWithContextInvalidHost tests the DialWithContext method with intentional breaking
-// for the Client object
-func TestClient_DialWithContextInvalidHost(t *testing.T) {
-	c, err := getTestConnection(true)
-	if err != nil {
-		t.Skipf("failed to create test client: %s. Skipping tests", err)
-	}
-	c.host = "invalid.addr"
-	ctx := context.Background()
-	if err = c.DialWithContext(ctx); err == nil {
-		t.Errorf("dial succeeded but was supposed to fail")
-		return
-	}
-}
 
 // TestClient_DialWithContextInvalidHELO tests the DialWithContext method with intentional breaking
 // for the Client object
@@ -3701,12 +3712,19 @@ func handleTestServerConnection(connection net.Conn, props *serverProps) {
 		return
 	}
 
+	for {
 		data, err := reader.ReadString('\n')
 		if err != nil {
-		return
+			break
 		}
-	if !strings.HasPrefix(data, "EHLO") && !strings.HasPrefix(data, "HELO") {
+		time.Sleep(time.Millisecond)
-		fmt.Printf("expected EHLO, got %q", data)
+
+		var datastring string
+		data = strings.TrimSpace(data)
+		switch {
+		case strings.HasPrefix(data, "EHLO"), strings.HasPrefix(data, "HELO"):
+			if len(strings.Split(data, " ")) != 2 {
+				_ = writeLine("501 Syntax: EHLO hostname")
 				return
 			}
 			if props.FailOnHelo {
@@ -3716,17 +3734,6 @@ func handleTestServerConnection(connection net.Conn, props *serverProps) {
 			if err = writeLine("250-localhost.localdomain\r\n" + props.FeatureSet); err != nil {
 				return
 			}
-
-	for {
-		data, err = reader.ReadString('\n')
-		if err != nil {
-			break
-		}
-		time.Sleep(time.Millisecond)
-
-		var datastring string
-		data = strings.TrimSpace(data)
-		switch {
 		case strings.HasPrefix(data, "MAIL FROM:"):
 			from := strings.TrimPrefix(data, "MAIL FROM:")
 			from = strings.ReplaceAll(from, "BODY=8BITMIME", "")