Server error (email)

I have started getting these error messages on a daily basis:

Code:

Symfony\Component\Mailer\Exception\TransportException: Email to *** from *** failed: Expected response code "250/251/252" but got code "554", with message "554 5.7.1 <unknown[2a02:4780:8:1225:0:2472:ae72:2]>: Client host rejected: Access denied". src/vendor/symfony/mailer/Transport/Smtp/SmtpTransport.php:307

What could be the reason behind?

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