[email protected]: host alt1.gmail-smtp-in.l.google.com[188.8.131.52] said: 450-4.2.1 The user you are trying to contact is receiving mail at a rate that 450-4.2.1 prevents additional messages from being delivered.
Please resend your 450-4.2.1 message at a later time. If the user is able to receive mail at that 450-4.2.1 time, your message will be delivered. For more information, please 450-4.2.1 visit 450 4.2.1 https://support.google.com/mail/?p=ReceivingRate w138si33038971ybg.321 - gsmtp (in reply to RCPT TO command)
The Gmail recipient is accepting mail at a limited rate. All emails above this rate are blocked.
Wait until the emails are delivered.
If you’re sending many emails at a time, or emails to large groups, review Gmail bulk senders guidelines.
If the issue persists and receiver should receive email, contact Gmail support at https://support.google.com/mail/.