Setup: group with private Group Guidelines.
Situation: Non-member sends message to the help address, gets back instructions that include sending email to group+guidelines address. Non-member sends email to that address and receives a copy of the group guidelines via email, which they should not be privy to.
Best solution IMO: the automated help message does not include instructions for getting the guidelines if the person is a non-member and the guidelines are private.
Alternate solution: keep help message the same but if a non-member sends a message to the guidelines address send back an error message ("this group's guidelines are only available to group members" yada yada).
Messages are the sole opinion of the author, especially the fishy ones.