Email error: 550 Sender rate overlimit

I got a slew of these email errors lately:

Code:

Symfony\Component\Mailer\Exception\TransportException: Email to <...> from <...> failed: Process failed with exit code 11: 550 Sender rate overlimit - 13.7 / 1m src/vendor/symfony/mailer/Transport/Smtp/Stream/ProcessStream.php:60

Anyone with an idea what may be causing this?

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