Email replies will go back directly to the email address you have in your account.
In other words, if you are sending emails from PhoneBurner, and you have not setup your SMTP server, when the person replies to the email you sent it will go to the address you've put in here (see below).
To navigate to this area:
1. Click on My Account > My Account in the top right hand corner. (https://www.phoneburner.com/myaccount/email_settings
2. Click Email and update the email settings