Is there any way to customize this to something more specific to our group and less technical?
No.
The only part of the message that comes from Groups.io is the numeric status code and the text one-liner. All the rest is boilerplate from the recipient's email service (often called "mailer daemon").
An improvement has been suggested in beta, but not acted on due to "reasons". You could chime in there in support of the suggestion.
There are ways to mitigate the risk of backscatter, but probably not without using a DMARC-like scheme to authenticate incoming message posts, something that Groups.io has been reluctant to do (but I think they should).
Shal
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