Remove redundant connection check in auth function

The checkConn call in the auth function was redundant because the connection is already managed appropriately elsewhere. Removing this unnecessary check simplifies the code and avoids potential duplication of error handling.

client.go

@@ -1094,10 +1094,6 @@ func (c *Client) DialAndSendWithContext(ctx context.Context, messages ...*Msg) e
 //   - An error if the connection check fails, if no supported authentication method is found,
 //     or if the authentication process fails.
 func (c *Client) auth() error {
-	if err := c.checkConn(); err != nil {
-		return fmt.Errorf("failed to authenticate: %w", err)
-	}
-
 	if c.smtpAuth == nil && c.smtpAuthType != SMTPAuthNoAuth {
 		hasSMTPAuth, smtpAuthType := c.smtpClient.Extension("AUTH")
 		if !hasSMTPAuth {