Symptoms
There are undelivered emails in the deferred state which can be found at Tools&Settings > Mail Server Settings > Mail Queue.
The following error can be found in the /var/log/maillog file:
postfix/smtp[32699]: EB5F461000A2: host gmail-smtp-in.l.google.com[203.0.113.2] 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 aabb1122ccddeeff.547 - gsmtp (in reply to RCPT TO command)
Cause
The Gmail recipient is accepting mail at a limited rate. All emails above this rate are blocked.
Resolution
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/.