Send Mail As using smtp.gmail.com not authenticating

Keep getting the error "Outgoing Authentication Failure for xxx@gmail.com" every time I try to add a gmail "send mail as" account in Zoho. As far as I can tell, I have everything enabled correctly on Gmail. I've run through the list https://support.google.com/mail/answer/78775?hl=en&ref_topic=3397501 and everything should be working. Are there any other ideas?