If someone tries to send an email to this_mailbox_is_full@domain.com and you are using domain.com as your SMTP server, it will generate a Mailbox unavailable error.
16:45:49 - Error sending email to this_mailbox_is_full@domain.com through Email gateway: Mailbox unavailable. The server response was: Request failed; Mailbox unavailable
16:45:49 - SHUTTING DOWN EMAIL GATEWAY. Too many retries (exceeded limit of 5 retries)
VarAC should not shut down the email gateway, as the gateway itself is still properly configured. The message should instead be discarded.