After sending out an email campaign you will receive lots of “reply-to” emails (such as unsubscribe or challenge-response requests, bounces and blocks). These are most generated automatically and go back to your sending address and not your bounce-handling address.
Based on analysis of recent distributions of EmailLabs’ newsletter "Intevation Report", these “reply-to” emails account for less than 2% of the total mailing list, but they can affect list integrity if you don't act on them promptly.
There are six types of email replies:
1. Out-of-office replies
3. Inactive addresses
4. Challenge-response requests
5. Blocks and bounce reports
6. Feedback, rendering problems and unsubscribe requests
In this article you can read how you should act on them.