Add isConnected flag to track active connection state

Introduced the isConnected boolean flag in the Client struct to clearly indicate whether there is an active connection. Updated relevant methods to set this flag accordingly, ensuring consistent state management across the Client's lifecycle.

smtp/smtp.go

@@ -67,6 +67,9 @@ type Client struct {
 	// helloError is the error from the hello
 	helloError error
 
+	// isConnected indicates if the Client has an active connection
+	isConnected bool
+
 	// localName is the name to use in HELO/EHLO
 	localName string // the name to use in HELO/EHLO
 
@@ -113,6 +116,7 @@ func NewClient(conn net.Conn, host string) (*Client, error) {
 	}
 	c := &Client{Text: text, conn: conn, serverName: host, localName: "localhost"}
 	_, c.tls = conn.(*tls.Conn)
+	c.isConnected = true
 
 	return c, nil
 }
@@ -121,6 +125,7 @@ func NewClient(conn net.Conn, host string) (*Client, error) {
 func (c *Client) Close() error {
 	c.mutex.Lock()
 	err := c.Text.Close()
+	c.isConnected = false
 	c.mutex.Unlock()
 	return err
 }
@@ -516,8 +521,7 @@ func (c *Client) Quit() error {
 	}
 	c.mutex.Lock()
 	err = c.Text.Close()
-	c.Text = nil
-	c.conn = nil
+	c.isConnected = false
 	c.mutex.Unlock()
 
 	return err
@@ -558,11 +562,12 @@ func (c *Client) SetDSNRcptNotifyOption(d string) {
 // Returns true if the `conn` field is not nil, indicating an active connection.
 func (c *Client) HasConnection() bool {
 	c.mutex.RLock()
-	conn := c.conn
+	isConn := c.isConnected
 	c.mutex.RUnlock()
-	return conn != nil
+	return isConn
 }
 
+// UpdateDeadline sets a new deadline on the SMTP connection with the specified timeout duration.
 func (c *Client) UpdateDeadline(timeout time.Duration) error {
 	c.mutex.Lock()
 	if err := c.conn.SetDeadline(time.Now().Add(timeout)); err != nil {