Since upgrading to 6.6.6, I see this log message every day around the same time:
Jan 8 06:00:13 stylophora sSMTP: Creating SSL connection to host Jan 8 06:00:13 stylophora sSMTP: SSL connection using ECDHE-RSA-CHACHA20-POLY1305 Jan 8 06:00:13 stylophora sSMTP: Authorization failed (535 5.7.8 https://support.google.com/mail/?p=BadCredentials g25sm28692448qki.29 - gsmtp)
I had never previously setup email notifications for anything so I went and took a look at the notifications page in settings and found that email notifications are all unchecked, however, the default gmail smtp server info is there and with no way to disable it.
There is no option to disable this in "preset service" and if you choose custom service and blank out all the options, it will not let you save the form. Hitting reset will revert to default gmail preset.
Expected outcome - like the rest of the notification services, being able to disable the notification types: