I have read your question. I had also faced such issue recently.
Now a day’s outgoing e-mail is the most sensitive matter and also it is treated as bulk or spam. Sending Bulk e-mails has also become sensitive towards the receivers by using the “report spam” button now a days people tend to be very lazy and they just want to “report spam” instead of just trying to unsubscribe from the recipient or request the recipient for the removal from the email list. Please note the Gmail is not the bulk e-mail service provider so by repeatedly sending the bulk messages can cause the problems to your Gmail Account. So it may be the case that you have reached the limit for sending the mail.