Bulk Emails failed with 421 4.7.0 Try again later

Question

I have a requirement to send bulk emails with in my organization. I am Java Mail API and sending the mails with below config.

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Answer 1

Yes, I think there is a per minute limit for sending SMPT messages via Gmail (around 80 in my experience that of others around 80-100). But it's not a 100% hard limit but some sort of soft limit that is enforced when Gmail thinks its due.

First, check your message queue in G Suite admin area. If the queue is huge and still growing you send too fast.

Some useful links:

• G Suite SMTP Error Reference
• Gmail Bandwidth limits for accounts
• Gmail sending limits in G Suite
• Bulk Senders Guidelines

There is no further explanation what's wrong when you suffer from the error

421, "4.7.0", Try again later, closing connection.

General recommendations:

• you could use Pooled SMTP like https://nodemailer.com/smtp/pooled/

If pooling is used then Nodemailer keeps a fixed amount of connections open and sends the next message once a connection becomes available. It is most useful when you have a large number of messages that you want to send in batches or your provider allows you to only use a small amount of parallel connections.

• do not send too fast
• do not send more than 50 emails at once
• do not assume Gmail is a local system. It's an external service that uses sophisticated methods to make email globally available extending a 42-year-old invention that is far from perfect.
• do not try stupid things or Gmail will punish you.

Moral: If sending a large amount of email, it's probably best to use your own email server.