It would be useful to be able to trigger an action in the campaign builder when a lead replies by email. The action should be triggered on a specific email address or subject line.
One use of this is to have leads reply by email and then Mautic sends them a lead magnet. The email reply is extremely useful because most email service providers will automatically white label the email address an email sent to. This means a higher delivery rate and better inbox on Gmail hopefully bypassing the Promotional box.
This is such a successful technic that several of the big ESPs offer this option now, Additionally, there are reply by email opt-in forms specifically for this.