I changed the setting for Google Groups email addresses to not save email history, so I’ll introduce the procedure.

Google Groups: Change "conversation history" to "off"

https://groups.google.com/a/your-domain.example.com/g/your-group/settings

Conversation history Archive conversations in Google Groups and send email digests of posted messages to members (required for collaborative inbox).

From here, change “conversation history” to “off”.

Google Groups: Delete all "conversations"

From https://groups.google.com/a/your-domain.example.com/g/your-group/,

delete all “conversations”.

That’s all from the Gemba, where I want to operate Google Groups email addresses securely.