def sender(recipients='[email protected],[email protected]'): 

    body = 'Your email content here'
    msg = MIMEMultipart()

    msg['Subject'] = 'Email Subject'
    msg['From'] = '[email protected]'
    msg['To'] = (', ').join(recipients.split(','))

    msg.attach(MIMEText(body,'plain'))

    server = smtplib.SMTP('smtp.gmail.com', 587)
    server.starttls()
    server.login('[email protected]', 'yourpassword')
    server.send_message(msg)
    server.quit()

It only worked for me with send_message function and using the join function in the list whith recipients, python 3.6.

import smtplib
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText

def sender(recipients): 

    body = 'Your email content here'
    msg = MIMEMultipart()

    msg['Subject'] = 'Email Subject'
    msg['From'] = '[email protected]'
    msg['To'] = (', ').join(recipients.split(','))

    msg.attach(MIMEText(body,'plain'))

    server = smtplib.SMTP('smtp.gmail.com', 587)
    server.starttls()
    server.login('[email protected]', 'yourpassword')
    server.send_message(msg)
    server.quit()

if __name__ == '__main__':
    sender('[email protected],[email protected]')

It only worked for me with send_message function and using the join function in the list whith recipients, python 3.6.