https://support.google.com/groups/answer/2627595?hl=en
I personally always disable it for public groups and then rely on Spam filtering of the recipient inbox.
By the way - the bounce message you are seeing is what is sent when the Spam Classification Server determined your message to be Spam. The main reasons for that are things like sending from a bad IP, having certain keywords that are strongly correlated with Spam, or having domains in your message that are associated with Spam (based on other messages having been marked as Spam containing those domains).
Google Apps SMTP -> Google Apps Group (different domain) == FAIL
Fastmail SMTP -> Google Apps Group (different domain) == SUCCESS?
It's been a long time since I looked at this. We did have great internal tools to look up the classification of individual messages, including the top 10 deciding reasons for a given message classification. With a paid Google Apps domain a support rep investigating your ticket would eventually look at this tool to figure out what's going on.
Yes, I pay Fastmail and I wasn't paying for Google Apps. But I also get responsive support from Fastmail, IMAP push to iOS devices, more flexibility in delivery rules, etc.