FIX: Do not add mailing list headers to group SMTP emails (PR #13431)

When we are emailing people from a group inbox, we are having a PM conversation with them, as a support account would. In this case mailing list headers do not make sense. It is not like a forum topic where you may have tens or hundreds of participants – it is a conversation between the group and a small handful of people directly contacting the group, often just one person.

The only header left in tact was List-Unsubsribe which is important for letting people opt out to notifications.

GitHub

Re: List-Unsubsribe, are you 100% sure we need this? When I email someone random I don’t need to do so, feels a bit odd to allow this here.

@SamSaffron I think it is good to keep because you could CC someone once and then they keep getting emails from the topic, but maybe they don’t care about it. Maybe we revisit that decision at some point and remove it if it seems fine?