450 4.2.1: recipient is receiving mail too fast

What it means

This deferral is about the recipient rather than about you. Gmail limits how quickly a single account can receive messages, and this mailbox is over that limit for the moment. Google documents two wordings: one says the user is receiving email too quickly, the other that the incoming rate prevents more messages from being delivered. Both ask you to resend later and say the message will be delivered once the account can accept mail again. Your server should keep the message queued and retry on its normal schedule. A mailbox caught in a mail loop, flooded by automated alerts or sitting on a busy distribution list is the typical case.

Common causes

• The recipient's mailbox is getting a burst of messages from many senders at once, such as alerts or list traffic.
• Your own system is sending that one recipient many messages in a short time, for example from a script.
• A forwarding or auto-reply loop is pushing a stream of messages into the account.

How to fix it

1. 01Let your server retry on its normal schedule: a 450 is a deferral, and Gmail delivers once the account can receive again.
2. 02If you send to this address in bulk or from an automated process, slow down and spread the messages over time.
3. 03Check your logs for a loop between your system and the recipient, such as auto-replies answering each other.
4. 04If the deferrals go on for days, contact the recipient another way so their admin can find what is flooding the mailbox.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• Gmail SMTP errors and codes (Google Workspace Admin Help)knowledge.workspace.google.com

Further reading

• Soft bounce vs hard bounce
• The standard meaning of 5.2.1: mailbox disabled