Microsoft Office 365 suffered from this problem for about a year and have since solved it.. now it seems it is the time for Google to suffer.
Spammers are creating Google Apps for Domains (now G-Suite) accounts and creating mailing groups and using them to send spam.
The “Report abuse” button on the spam email (appended by Google when the message is processed) results in an error:
This group is on a private domain.
Please sign in with an authorised account to view this content.
Attempts to sign in won’t work as you need an account on the custom domain.
Google are notoriously difficult to communicate with if their automated “report via here” buttons don’t work.. so I expect they are even unaware of most of the domains being used abusively like this.
The domains that I’ve seen use this are: