Refactor codebase to align with updated Go version support

client.go

@@ -1209,6 +1209,55 @@ func (c *Client) Send(messages ...*Msg) (returnErr error) {
 	return c.SendWithSMTPClient(c.smtpClient, messages...)
 }
 
+// SendWithSMTPClient attempts to send one or more Msg using a provided smtp.Client with an
+// established connection to the SMTP server. If the smtp.Client has no active connection to
+// the server, SendWithSMTPClient will fail with an error. For each of the provided Msg, it
+// will associate a SendError with the Msg in case of a transmission or delivery error.
+//
+// This method first checks for an active connection to the SMTP server. If the connection is
+// not valid, it returns a SendError. It then iterates over the provided messages, attempting
+// to send each one. If an error occurs during sending, the method records the error and
+// associates it with the corresponding Msg. If multiple errors are encountered, it aggregates
+// them into a single SendError to be returned.
+//
+// Parameters:
+//   - client: A pointer to the smtp.Client that holds the connection to the SMTP server
+//   - messages: A variadic list of pointers to Msg objects to be sent.
+//
+// Returns:
+//   - An error that represents the sending result, which may include multiple SendErrors if
+//     any occurred; otherwise, returns nil.
+func (c *Client) SendWithSMTPClient(client *smtp.Client, messages ...*Msg) (returnErr error) {
+	escSupport := false
+	if client != nil {
+		escSupport, _ = client.Extension("ENHANCEDSTATUSCODES")
+	}
+	if err := c.checkConn(client); err != nil {
+		returnErr = &SendError{
+			Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err),
+			errcode: errorCode(err), enhancedStatusCode: enhancedStatusCode(err, escSupport),
+		}
+		return
+	}
+
+	var errs []error
+	defer func() {
+		returnErr = errors.Join(errs...)
+	}()
+
+	for id, message := range messages {
+		if message == nil {
+			continue
+		}
+		if sendErr := c.sendSingleMsg(client, message); sendErr != nil {
+			messages[id].sendError = sendErr
+			errs = append(errs, sendErr)
+		}
+	}
+
+	return
+}
+
 // auth attempts to authenticate the client using SMTP AUTH mechanisms. It checks the connection,
 // determines the supported authentication methods, and applies the appropriate authentication
 // type. An error is returned if authentication fails.

client_test.go

@@ -3652,6 +3652,109 @@ func TestClient_sendSingleMsg(t *testing.T) {
 	})
 }
 
+func TestNewClientNewVersionsOnly(t *testing.T) {
+	tests := []struct {
+		name       string
+		option     Option
+		expectFunc func(c *Client) error
+		shouldfail bool
+		expectErr  *error
+	}{
+		{
+			"WithLogger log.JSONlog", WithLogger(log.NewJSON(os.Stderr, log.LevelDebug)),
+			func(c *Client) error {
+				if c.logger == nil {
+					return errors.New("failed to set logger. Want logger bug got got nil")
+				}
+				loggerType := reflect.TypeOf(c.logger).String()
+				if loggerType != "*log.JSONlog" {
+					return fmt.Errorf("failed to set logger. Want logger type: %s, got: %s",
+						"*log.JSONlog", loggerType)
+				}
+				return nil
+			},
+			false, nil,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			client, err := NewClient(DefaultHost, tt.option)
+			if !tt.shouldfail && err != nil {
+				t.Fatalf("failed to create new client: %s", err)
+			}
+			if tt.shouldfail && err == nil {
+				t.Errorf("client creation was supposed to fail, but it didn't")
+			}
+			if tt.shouldfail && tt.expectErr != nil {
+				if !errors.Is(err, *tt.expectErr) {
+					t.Errorf("error for NewClient mismatch. Expected: %s, got: %s",
+						*tt.expectErr, err)
+				}
+			}
+			if tt.expectFunc != nil {
+				if err = tt.expectFunc(client); err != nil {
+					t.Errorf("NewClient with custom option failed: %s", err)
+				}
+			}
+		})
+	}
+}
+
+func TestClient_DialWithContextNewVersionsOnly(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	PortAdder.Add(1)
+	serverPort := int(TestServerPortBase + PortAdder.Load())
+	featureSet := "250-AUTH PLAIN\r\n250-8BITMIME\r\n250-DSN\r\n250 SMTPUTF8"
+	go func() {
+		if err := simpleSMTPServer(ctx, t, &serverProps{FeatureSet: featureSet, ListenPort: serverPort}); err != nil {
+			t.Errorf("failed to start test server: %s", err)
+			return
+		}
+	}()
+	time.Sleep(time.Millisecond * 30)
+	t.Run("connect with full debug logging and auth logging", func(t *testing.T) {
+		ctxDial, cancelDial := context.WithTimeout(ctx, time.Millisecond*500)
+		t.Cleanup(cancelDial)
+
+		logBuffer := bytes.NewBuffer(nil)
+		client, err := NewClient(DefaultHost, WithPort(serverPort), WithTLSPolicy(NoTLS),
+			WithDebugLog(), WithLogAuthData(), WithLogger(log.NewJSON(logBuffer, log.LevelDebug)),
+			WithSMTPAuth(SMTPAuthPlain), WithUsername("test"), WithPassword("password"))
+		if err != nil {
+			t.Fatalf("failed to create new client: %s", err)
+		}
+
+		if err = client.DialWithContext(ctxDial); err != nil {
+			var netErr net.Error
+			if errors.As(err, &netErr) && netErr.Timeout() {
+				t.Skip("failed to connect to the test server due to timeout")
+			}
+			t.Fatalf("failed to connect to the test server: %s", err)
+		}
+		t.Cleanup(func() {
+			if err = client.Close(); err != nil {
+				t.Errorf("failed to close the client: %s", err)
+			}
+		})
+
+		logs := parseJSONLog(t, logBuffer)
+		if len(logs.Lines) == 0 {
+			t.Errorf("failed to enable debug logging, but no logs were found")
+		}
+		authFound := false
+		for _, logline := range logs.Lines {
+			if strings.EqualFold(logline.Message, "AUTH PLAIN AHRlc3QAcGFzc3dvcmQ=") &&
+				logline.Direction.From == "client" && logline.Direction.To == "server" {
+				authFound = true
+			}
+		}
+		if !authFound {
+			t.Errorf("logAuthData not working, no authentication info found in logs")
+		}
+	})
+}
+
 func TestClient_checkConn(t *testing.T) {
 	t.Run("connection is alive", func(t *testing.T) {
 		ctx, cancel := context.WithCancel(context.Background())

log/jsonlog.go

@@ -2,9 +2,6 @@
 //
 // SPDX-License-Identifier: MIT
 
-//go:build go1.21
-// +build go1.21
-
 package log
 
 import (

log/jsonlog_test.go

@@ -2,9 +2,6 @@
 //
 // SPDX-License-Identifier: MIT
 
-//go:build go1.21
-// +build go1.21
-
 package log
 
 import (