Can you switch to React app and go to settings => system logs, it should show the logs for failed events.
Unfortunately no exact email / client reference there.
I think you are actually getting the 421 reply from google SMPT - same as I do, seems like something is not handling the email queue on the invoice ninja side correctly. 421 response in oure case should mean too many connections.
See example below with my fails:
root:"Expected response code "250" but got code "421", with message "421-4.7.0 Try again later, closing connection. (EHLO) 421-4.7.0 For more information, go to 421 4.7.0 https://support.google.com/a/answer/3221692 l19-20020a056638145300b0046edaf89690sm58018jad.46 - gsmtp"."
I’ve set my Google SMTP using - How to send email from your Google Workplace hosted email account
I also can send 1000 emails in a raw using google scripts and google spreadsheet, but can’t send single email with invoice ninja 
Reason I think something happened on invoice ninja side recently.