In Google cloud function, have FileNotFoundError: [Errno 2] No such file or directory: '/usr/sbin/sendmail': '/usr/sbin/sendmail'

Question

I want to send email with attachments using a python script in Google Cloud Function Here is the code:

def send_mail(text, from_user, to_user, subject):
    print ("SENDING EMAIL....")
    msg = MIMEMultipart()
    msg['From'] = from_user
    msg['To'] = ", ".join(to_user)
    msg['Subject'] = subject
    msg.attach(MIMEText(text, 'plain'))
    p = MIMEBase('application', 'octet-stream')
    attachment = open(join_path(OUTPUT_FILE_DIR, OUTPUT_FILENAME), "rb")
    p.add_header('Content-Disposition', "attachment; filename= %s"
                 % OUTPUT_FILENAME)
    p.set_payload(attachment.read())
    encoders.encode_base64(p)
    msg.attach(p)
    attachment.close()
    proc = Popen(['/usr/sbin/sendmail', '-t'], stdin=PIPE)
    proc.communicate(msg.as_string())
    return

And got this error: FileNotFoundError: [Errno 2] No such file or directory: '/usr/sbin/sendmail': '/usr/sbin/sendmail'

Is this proc legal in gcp? or Is there any replacement for it? Thank you so much