Yep, the mail server needs to support plus aliases (email@example.com will point to firstname.lastname@example.org).
Is there any way to ensure that the email recipients of the new topic notifications include everyone within the group it was sent to (as opposed to sending it like it seems it’s only going to one individual)?
You mean adding all recipients into the TO or CC fields? Mail servers frown upon that and the messages will eventually be flagged as spam. Not to mention that everyone will be able to see everyone else’s addresses, which is bad privacy-wise.
That’s why we stick to one email per recipient (we actually do send one email per recipient, it doesn’t just seem like it =) ).
Sorry I can’t help you on that front.