If they don't reply to the challenge in a timely manner, their mail will be treated as spam, otherwise it is re-delivered to your Inbox. The general assumption is that spammers either use fake addresses or don't reply to the challenge while senders of good mail do.
Still, the challenging message is an annoyance for new senders (and newsletter administrators in particular) and generates a lot of unnecessary email traffic. Fortunately, it's easy to turn off challenging completely in Matador.
Turn Off Challenging Senders in MailFrontier Desktop
To prevent MailFrontier Desktop from challenging new senders:
- Click on the Matador system tray icon with the right mouse button.
- Select Open Matador from MailFrontier... from the menu.
- Go to the Challenges category.
- Make sure the Challenges slider is set to Never.
- Click Close.