here a link to some information on troubleshooting Google code error. About SMTP error messages - Google Workspace Admin Help
More detail on the code 421
It looks like your ip address is temporarily blocked from using Google SMTP service. It’s possible the Google SMTP system thinks somehow your system was compromised and spamming so it’s blocking the ip from using it further.
Since the code page states it’s temporary, I would take this moment to check to see how many emails are you allow to send per seconds / minutes / hours and create a batch sending in cronjob to prevent triggering Google SMTP into a meltdown mode thinking you are a spammer.