Add comprehensive unit tests for email address getters

Introduced unit tests for GetFrom, GetFromString, GetTo, GetToString, GetCc, GetCcString, GetBcc, and GetBccString methods. These tests cover scenarios with single, multiple, and no addresses, ensuring correct functionality of email address retrieval methods.

client.go

@@ -1261,14 +1261,13 @@ func (c *Client) sendSingleMsg(message *Msg) error {
 			affectedMsg: message,
 		}
 	}
-	message.isDelivered = true
-
 	if err = writer.Close(); err != nil {
 		return &SendError{
 			Reason: ErrSMTPDataClose, errlist: []error{err}, isTemp: isTempError(err),
 			affectedMsg: message,
 		}
 	}
+	message.isDelivered = true
 
 	if err = c.Reset(); err != nil {
 		return &SendError{

client_test.go

@@ -3527,6 +3527,9 @@ func parseJSONLog(t *testing.T, buf *bytes.Buffer) logData {
 func testMessage(t *testing.T) *Msg {
 	t.Helper()
 	message := NewMsg()
+	if message == nil {
+		t.Fatal("failed to create new message")
+	}
 	if err := message.From(TestSenderValid); err != nil {
 		t.Errorf("failed to set sender address: %s", err)
 	}

msg.go

@@ -847,7 +847,16 @@ func (m *Msg) CcIgnoreInvalid(rcpts ...string) {
 // References:
 //   - https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.3
 func (m *Msg) CcFromString(rcpts string) error {
-	return m.Cc(strings.Split(rcpts, ",")...)
+	src := strings.Split(rcpts, ",")
+	var dst []string
+	for _, address := range src {
+		address = strings.TrimSpace(address)
+		if address == "" {
+			continue
+		}
+		dst = append(dst, address)
+	}
+	return m.Cc(dst...)
 }
 
 // Bcc sets one or more "BCC" (blind carbon copy) addresses in the mail body for the Msg.
@@ -933,7 +942,16 @@ func (m *Msg) BccIgnoreInvalid(rcpts ...string) {
 // References:
 //   - https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.3
 func (m *Msg) BccFromString(rcpts string) error {
-	return m.Bcc(strings.Split(rcpts, ",")...)
+	src := strings.Split(rcpts, ",")
+	var dst []string
+	for _, address := range src {
+		address = strings.TrimSpace(address)
+		if address == "" {
+			continue
+		}
+		dst = append(dst, address)
+	}
+	return m.Bcc(dst...)
 }
 
 // ReplyTo sets the "Reply-To" address for the Msg, specifying where replies should be sent.
@@ -1069,8 +1087,7 @@ func (m *Msg) SetBulk() {
 //   - https://datatracker.ietf.org/doc/html/rfc5322#section-3.3
 //   - https://datatracker.ietf.org/doc/html/rfc1123
 func (m *Msg) SetDate() {
-	now := time.Now().Format(time.RFC1123Z)
-	m.SetGenHeader(HeaderDate, now)
+	m.SetDateWithValue(time.Now())
 }
 
 // SetDateWithValue sets the "Date" header for the Msg using the provided time value in a valid RFC 1123 format.
@@ -1170,6 +1187,9 @@ func (m *Msg) IsDelivered() bool {
 // References:
 //   - https://datatracker.ietf.org/doc/html/rfc8098
 func (m *Msg) RequestMDNTo(rcpts ...string) error {
+	if m.genHeader == nil {
+		m.genHeader = make(map[Header][]string)
+	}
 	var addresses []string
 	for _, addrVal := range rcpts {
 		address, err := mail.ParseAddress(addrVal)
@@ -1178,9 +1198,7 @@ func (m *Msg) RequestMDNTo(rcpts ...string) error {
 		}
 		addresses = append(addresses, address.String())
 	}
-	if _, ok := m.genHeader[HeaderDispositionNotificationTo]; ok {
 	m.genHeader[HeaderDispositionNotificationTo] = addresses
-	}
 	return nil
 }
 
@@ -1219,11 +1237,11 @@ func (m *Msg) RequestMDNAddTo(rcpt string) error {
 		return fmt.Errorf(errParseMailAddr, rcpt, err)
 	}
 	var addresses []string
-	addresses = append(addresses, m.genHeader[HeaderDispositionNotificationTo]...)
-	addresses = append(addresses, address.String())
-	if _, ok := m.genHeader[HeaderDispositionNotificationTo]; ok {
-		m.genHeader[HeaderDispositionNotificationTo] = addresses
+	if current, ok := m.genHeader[HeaderDispositionNotificationTo]; ok {
+		addresses = current
 	}
+	addresses = append(addresses, address.String())
+	m.genHeader[HeaderDispositionNotificationTo] = addresses
 	return nil
 }