Recently discovered in our Google Admin Portal that: Per User outbound Gateway is turned on, checking with other people I work with, no one knows why its turned on and when it was turned on.
I have been digging through the Google admin sdk and have not found a way to check to see if any users is using this setting.
In short, does anyone know how I can check to see if a user in our domain has a secondary smtp connection or a "Send Mail As" option in use?
So, the setting 'allow per user outbound gateway' does indeed give users permissions to add a 'From' address external to your domain. However if anyone is using this, they'll likely need to configure it as a send-mail as address inside Gmail.
You can list all 'Send as addresses' via the Email settings API as per https://developers.google.com/admin-sdk/email-settings/#retrieving_send-as_alias_settings. Note that the code examples provided will not work as they're out of date and showing client Login which has been disabled for months now. I'd be advising you to Auth with Oauth 2.0, call the endpoint with wget or CURL(probably wget since recursive)