This is the bump email sent from Outreach for the Reminder feature in the Outlook extension and add-in tool. The reminder feature allows you to set a bump reminder email for mailings to prospects that aren't replied to in a configured time limit. This can either bump that email in your inbox to remind you, and/or it can create a task if there is no response from the prospect.
You can set this up separately for different emails by clicking on the bell icon in the Outreach tool. If you want this automatically enabled for all of your messages, you can enable the Automatic Reminders option under the extension/add-in's cogwheel settings menu.