diff --git a/smtp/smtp.go b/smtp/smtp.go
index 787006c..1f1d603 100644
--- a/smtp/smtp.go
+++ b/smtp/smtp.go
@@ -30,6 +30,7 @@ import (
 	"net/textproto"
 	"os"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/wneessen/go-mail/log"
@@ -70,6 +71,10 @@ type Client struct {
 	// logger will be used for debug logging
 	logger log.Logger
 
+	// mutex is used to synchronize access to shared resources, ensuring that only one goroutine can access
+	// the resource at a time.
+	mutex sync.RWMutex
+
 	// tls indicates whether the Client is using TLS
 	tls bool
 
@@ -112,6 +117,9 @@ func NewClient(conn net.Conn, host string) (*Client, error) {
 
 // Close closes the connection.
 func (c *Client) Close() error {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
 	return c.Text.Close()
 }
 
@@ -139,12 +147,19 @@ func (c *Client) Hello(localName string) error {
 	if c.didHello {
 		return errors.New("smtp: Hello called after other methods")
 	}
+
+	c.mutex.Lock()
 	c.localName = localName
+	c.mutex.Unlock()
+
 	return c.hello()
 }
 
 // cmd is a convenience function that sends a command and returns the response
 func (c *Client) cmd(expectCode int, format string, args ...interface{}) (int, string, error) {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
 	c.debugLog(log.DirClientToServer, format, args...)
 	id, err := c.Text.Cmd(format, args...)
 	if err != nil {
@@ -160,7 +175,10 @@ func (c *Client) cmd(expectCode int, format string, args ...interface{}) (int, s
 // helo sends the HELO greeting to the server. It should be used only when the
 // server does not support ehlo.
 func (c *Client) helo() error {
+	c.mutex.Lock()
 	c.ext = nil
+	c.mutex.Unlock()
+
 	_, _, err := c.cmd(250, "HELO %s", c.localName)
 	return err
 }
@@ -175,9 +193,13 @@ func (c *Client) StartTLS(config *tls.Config) error {
 	if err != nil {
 		return err
 	}
+
+	c.mutex.Lock()
 	c.conn = tls.Client(c.conn, config)
 	c.Text = textproto.NewConn(c.conn)
 	c.tls = true
+	c.mutex.Unlock()
+
 	return c.ehlo()
 }
 
@@ -185,6 +207,9 @@ func (c *Client) StartTLS(config *tls.Config) error {
 // The return values are their zero values if [Client.StartTLS] did
 // not succeed.
 func (c *Client) TLSConnectionState() (state tls.ConnectionState, ok bool) {
+	c.mutex.RLock()
+	defer c.mutex.RUnlock()
+
 	tc, ok := c.conn.(*tls.Conn)
 	if !ok {
 		return
@@ -249,7 +274,9 @@ func (c *Client) Auth(a Auth) error {
 				// abort the AUTH. Not required for XOAUTH2
 				_, _, _ = c.cmd(501, "*")
 			}
+			c.mutex.Lock()
 			_ = c.Quit()
+			c.mutex.Unlock()
 			break
 		}
 		if resp == nil {
@@ -275,6 +302,8 @@ func (c *Client) Mail(from string) error {
 		return err
 	}
 	cmdStr := "MAIL FROM:<%s>"
+
+	c.mutex.RLock()
 	if c.ext != nil {
 		if _, ok := c.ext["8BITMIME"]; ok {
 			cmdStr += " BODY=8BITMIME"
@@ -287,6 +316,8 @@ func (c *Client) Mail(from string) error {
 			cmdStr += fmt.Sprintf(" RET=%s", c.dsnmrtype)
 		}
 	}
+	c.mutex.RUnlock()
+
 	_, _, err := c.cmd(250, cmdStr, from)
 	return err
 }
@@ -298,7 +329,11 @@ func (c *Client) Rcpt(to string) error {
 	if err := validateLine(to); err != nil {
 		return err
 	}
+
+	c.mutex.RLock()
 	_, ok := c.ext["DSN"]
+	c.mutex.RUnlock()
+
 	if ok && c.dsnrntype != "" {
 		_, _, err := c.cmd(25, "RCPT TO:<%s> NOTIFY=%s", to, c.dsnrntype)
 		return err
@@ -423,6 +458,9 @@ func (c *Client) Extension(ext string) (bool, string) {
 		return false, ""
 	}
 	ext = strings.ToUpper(ext)
+
+	c.mutex.RLock()
+	defer c.mutex.RUnlock()
 	param, ok := c.ext[ext]
 	return ok, param
 }
@@ -497,6 +535,9 @@ func (c *Client) HasConnection() bool {
 }
 
 func (c *Client) UpdateDeadline(timeout time.Duration) error {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
 	if err := c.conn.SetDeadline(time.Now().Add(timeout)); err != nil {
 		return fmt.Errorf("smtp: failed to update deadline: %w", err)
 	}
diff --git a/smtp/smtp_ehlo.go b/smtp/smtp_ehlo.go
index ae80a62..457be57 100644
--- a/smtp/smtp_ehlo.go
+++ b/smtp/smtp_ehlo.go
@@ -25,6 +25,9 @@ func (c *Client) ehlo() error {
 	if err != nil {
 		return err
 	}
+
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
 	ext := make(map[string]string)
 	extList := strings.Split(msg, "\n")
 	if len(extList) > 1 {
diff --git a/smtp/smtp_ehlo_117.go b/smtp/smtp_ehlo_117.go
index c516a36..c40297f 100644
--- a/smtp/smtp_ehlo_117.go
+++ b/smtp/smtp_ehlo_117.go
@@ -28,6 +28,9 @@ func (c *Client) ehlo() error {
 	if err != nil {
 		return err
 	}
+
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
 	ext := make(map[string]string)
 	extList := strings.Split(msg, "\n")
 	if len(extList) > 1 {