Challenge Response is a method some email providers use to block spammers. If someone attempts to send an email from an email address that is not in the users address book, a challenge email is sent back to the sender. The challenge email will require the sender to verify that they are a human, and not a computer program. Typically this is done using a CAPTCHA code.
The user is asked to enter the captcha code (8B7E in this case) into a text box to validate that they are a human, and not an automated sender.
XMS Email Application Server's bounce management filter separates challenge response emails and allows you to forward them to another email address (such as your secretary) so that they can be processed by a human, and your newsletter will be delivered to users with challenge response accounts.
Overall the challenge response filter can increase the deliverability of your email campaign.