Entirely unrelated to the Slack discussion, but I wonder if that could be a positive pattern for a mail client: only allowing you to define the recipients when you have written out the full message.
Also, when replying to a long mail thread, showing you the previous list of recipients and requiring you to select those that you want to include in your next mail. Could limit CC sprawl quite a bit.
Also, when replying to a long mail thread, showing you the previous list of recipients and requiring you to select those that you want to include in your next mail. Could limit CC sprawl quite a bit.