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450 4.2.1 error: recipient receiving mail too fast

Gmail's 450 4.2.1 means that one recipient is getting mail faster than Gmail allows, so it's deferring yours. It's temporary; space out your sends.

Their mailboxSomething on their side, like a full mailbox. Usually not about you.

What 450 4.2.1 means

Gmail450 4.2.1 The user you are trying to contact is receiving email too quickly.

The mailbox is over Gmail's inbound rate, so delivery waits.

List problem or sender problem?

Their mailbox. It's the recipient's inbound limit, though your own sends to that one person count toward it. It's a temporary failure: the sending server retries on its own, but repeated deferrals are a warning.

How to fix 450 4.2.1

  1. Let the retry happen; it usually goes through later.
  2. Don't send several messages to the same person in quick succession.

In SendingOS, 450 4.2.1 shows on the conversation with its reason and code, so a problem on their side never gets mistaken for one on yours.

Not sure whether a spike in bounces is the list or the domain? Read High email bounce rate? Burned domain or bad list.

452 4.2.2421 4.7.28550 5.2.1

Sources: Google: Gmail SMTP errors and codes