Sorry for the late reply.
This is a hosting issue.
something is blocking the connection, maybe spam filter, etc…
One thing to check is that your host doesn’t have throttling enabled on your mail server. i.e. your server might be trying to send emails too quickly. Some hosts throttle the number of emails that can be sent per sec/min/hour.
Or, it could be the number of connections being made from your server to the mail server, i.e. if your server is opening up too many concurrent connections (sending multiple emails in quick succession).
With GSuite, I’m not 100% certain on the limits, but I have seen issues in the past whereby a clients App was attempting to connect and login way too quickly which was causing the connections to be rejected. After a period of time, he could connect again. There was a difference between different level of GSuite accounts. I’ll see if I can dig out any references to this.
Are you able to see the actual error code returned by GSuite in your logs?
Failing that, if you log into GSuite, you should be able to see connection requests and possibly any errors (again I can’t remember where that is in GSuite off the top of my head but if you have no joy locating it, I’ll try to find the reference).
The PHPMailer issue you link to (stackoverflow) would be a hard error rather than intermittent.
Just a few ideas…
