What is an inbox in Slack?
An inbox in Slack is a channel connected to an external platform like an email, Facebook, or Twitter account.
By using MailClark to link an external account to a Slack channel (the channel can be existing or new), you've just created an inbox in Slack! :)
Example: set up an inbox in #contact to answer emails sent to firstname.lastname@example.org and tweets to @YourCompany.
To learn how to create an inbox, read this question.