I have just sent an email with gmail through Python. Try to use smtplib.SMTP_SSL to make the connection. Also, you may try to change the gmail domain and port.
So, you may get a chance with:
server = smtplib.SMTP_SSL('smtp.googlemail.com', 465)
server.login(gmail_user, password)
server.sendmail(gmail_user, TO, BODY)
As a plus, you could check the email builtin module. In this way, you can improve the readability of you your code and handle emails headers easily.