Normally, you can't reply to contacts who responded to your emails inside Prospectreach. The software is ultimately used for cold-emailing outreach and lead generation. To respond to your prospects, you can use your own business or personal email, or you can access your Prospectreach email account's webmail dashboard and respond to the user from there.
Additionally, you can set up email forwarding from your Prospect Reach email sender account/s to your existing personal email account. In this way, replies to your email sender accounts in Prospect Reach will show up in your personal email account and you can reply from there or open up your email senders' webmail and reply from there.
You can search for instructions online in setting up email forwarding.
Here's an article about Cpanel Email forwarding - https://www.domains.co.za/knowledgebase/67/How-to-setup-email-forwarding-in-cPanel.html