“Nobody ever responds to spammers or to email they don’t care about, they simply ignore or delete it. So if you’re getting responses to your campaigns then it’s an obvious signal for email providers that these are legitimate emails people care about.”
Email providers like Gmail and Yahoo! are always on the lookout for Spam indicators. One way they do this is by monitoring email responses. If your emails never get a response, this can look a lot like Spam and decrease deliverability. Encourage users to reply to your messages and avoid using firstname.lastname@example.org type email addresses.