Since there is no owner/moderator activity involved, it would be better to have it read something like "system@groups.io" as sender
My suggestion would be:
From: The Groups.io Team <groupname+owner@groups.io>
That would use a Display Name reflecting the signature within the message, while preserving the From address so that any reply from the (former) member will go to the group owners/mods.
In most email interfaces I've used the Display Name is more prominent than the actual address, so I think this would address the concern.
