diff --git a/smtp/smtp.go b/smtp/smtp.go
index 1f1d603..379f5fe 100644
--- a/smtp/smtp.go
+++ b/smtp/smtp.go
@@ -118,9 +118,9 @@ 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()
+	err := c.Text.Close()
+	c.mutex.Unlock()
+	return err
 }
 
 // hello runs a hello exchange if needed.
@@ -158,17 +158,18 @@ func (c *Client) Hello(localName string) error {
 // 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 {
+		c.mutex.Unlock()
 		return 0, "", err
 	}
 	c.Text.StartResponse(id)
-	defer c.Text.EndResponse(id)
 	code, msg, err := c.Text.ReadResponse(expectCode)
 	c.debugLog(log.DirServerToClient, "%d %s", code, msg)
+	c.Text.EndResponse(id)
+	c.mutex.Unlock()
 	return code, msg, err
 }
 
@@ -208,13 +209,14 @@ func (c *Client) StartTLS(config *tls.Config) error {
 // 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
 	}
-	return tc.ConnectionState(), true
+	state, ok = tc.ConnectionState(), true
+	c.mutex.RUnlock()
+	return
 }
 
 // Verify checks the validity of an email address on the server.
@@ -274,9 +276,7 @@ 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 {
@@ -347,12 +347,23 @@ type dataCloser struct {
 	io.WriteCloser
 }
 
+// Close releases the lock, closes the WriteCloser, waits for a response, and then returns any error encountered.
 func (d *dataCloser) Close() error {
+	d.c.mutex.Lock()
 	_ = d.WriteCloser.Close()
 	_, _, err := d.c.Text.ReadResponse(250)
+	d.c.mutex.Unlock()
 	return err
 }
 
+// Write writes data to the underlying WriteCloser while ensuring thread-safety by locking and unlocking a mutex.
+func (d *dataCloser) Write(p []byte) (n int, err error) {
+	d.c.mutex.Lock()
+	n, err = d.WriteCloser.Write(p)
+	d.c.mutex.Unlock()
+	return
+}
+
 // Data issues a DATA command to the server and returns a writer that
 // can be used to write the mail headers and body. The caller should
 // close the writer before calling any more methods on c. A call to
@@ -362,7 +373,14 @@ func (c *Client) Data() (io.WriteCloser, error) {
 	if err != nil {
 		return nil, err
 	}
-	return &dataCloser{c, c.Text.DotWriter()}, nil
+	datacloser := &dataCloser{}
+
+	c.mutex.Lock()
+	datacloser.c = c
+	datacloser.WriteCloser = c.Text.DotWriter()
+	c.mutex.Unlock()
+
+	return datacloser, nil
 }
 
 var testHookStartTLS func(*tls.Config) // nil, except for tests
@@ -460,8 +478,8 @@ func (c *Client) Extension(ext string) (bool, string) {
 	ext = strings.ToUpper(ext)
 
 	c.mutex.RLock()
-	defer c.mutex.RUnlock()
 	param, ok := c.ext[ext]
+	c.mutex.RUnlock()
 	return ok, param
 }
 
@@ -494,7 +512,11 @@ func (c *Client) Quit() error {
 	if err != nil {
 		return err
 	}
-	return c.Text.Close()
+	c.mutex.Lock()
+	err = c.Text.Close()
+	c.mutex.Unlock()
+
+	return err
 }
 
 // SetDebugLog enables the debug logging for incoming and outgoing SMTP messages
@@ -536,11 +558,10 @@ 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)
 	}
+	c.mutex.Unlock()
 	return nil
 }