Understanding Reply Detection

Every 10 minutes, QuickMail automatically scans inboxes for Replies, Out-of-Office, and Bounce notifications.

This means sometimes, you'll notice a Reply before QuickMail does. Just give it a bit of time, and it'll be detected automatically!

Note: QuickMail will not be able to detect deleted replies.
If a Reply or Bounce is detected, QuickMail will automatically tag the Prospect on the Journey list for the Campaign, and all subsequent Follow-Ups will be cancelled:

Additionally, on the Prospect List, QuickMail will turn Replied Prospects into Valid prospects. 
Bounced prospects will be marked as invalid.

Quickmail will not send any further emails to a Bounced prospect, even if they are added to a new campaign.
If you'd prefer not to wait for QuickMail's hourly check to automatically detect a Reply, it's possible to manually tell QuickMail to give the Inbox a look.
Head to Settings > Inboxes, and click on the Inbox in question. Scroll to the bottom of the page, and use the Check Replies now button: