Gmail SMTP error - Temporary block?

Question

I am using Gmail servers to send email from my system, with a program. Recently I started getting errors like this:

Data command failed: 421 4.7.0 T

Answer 1

If you are using your free Gmail account to send bulk emails your are likely to see this kind of responses early on as the service is not intended to send application transaction messages, newsletters etc., event to subscribers that has opted in. The IMAP/SMTP service provided is for you to be able to use an email client like Microsoft Outlook with your Gmail account.

If you need to send transaction messages, I suggest you google "AWS SES" for starters.