I have tried finding the answer, but it didn't exist.
Nothing appears to happen until you reach close to 105% utilisation (e.g., ), at which point, messages get temporarily rejected as follows, after the
RCPT TO command is issued:
452-4.2.2 The email account that you tried to reach is over quota. Please direct 452-4.2.2 the recipient to 452 4.2.2 https://support.google.com/mail/?p=OverQuotaTemp XXXXXXXXXXXXXXXXXXX - gsmtp
Here's the link as a hyperlink: https://support.google.com/mail/?p=OverQuotaTemp