Email that can't reach Slack goes to the inbox owner

Sometimes an email can't be posted to Slack: the inbox has reached its plan's monthly limit, or the channel was archived.

Now that email goes to the inbox owner, with a note saying why it didn't reach Slack and a link to fix it. Replying to that email answers the customer directly.

At the monthly limit, the channel also gets one notice a day instead of one for every email.