I'm using Python to send mass emails. It seems that I send too many and too fast that I am getting SMTPRecipientsRefused(senderrs) errors. 
I used 

server = smtplib.SMTP('smtp.gmail.com', 587)
server.starttls()

Any ideas? Thanks!
It turns out that I was sending to empty recepients.
Getting SMTPRecipientsRefused(senderrs) because of sending too many?