with smtplib.SMTP(smtp_host, smtp_port) as server:
    for _ in range(5):
        try:
            server.ehlo()
            try:
                server.starttls()
                server.ehlo()
            except smtplib.SMTPNotSupportedError:
                pass
            server.login(smtp_user, smtp_password)
            server.sendmail(
                msg['From'],
                [msg['To']],
                msg.as_string()
            )
            isSuccess = True
            break
        except Exception as ex:
            print(ex)
            time.sleep(0.5)