Issue Blacklists for mailgroups not working

[ciB]

Hi,

I was planning on using mailgroups to easily send mails to a bunch of people in a team. Since this is for internal communication only, I was looking for a way to black all external mail that might be send to that mailgroup (spam).

But when adding "*@*.*" to the blacklist for that specific mailgroup, all inbound mail is still forwarded to every group member. I suspect the filter only works for content in the mailbox, not for forwarded mails.
Is there a nice workaround for my case? Or is this simply a bug?

ciB