Wing Commander Fowler, I suspect that Mailblocks will send a challenge message to the postmaster as it would to any sender not in your address book. The postmaster may or may not choose to respond and let the message through. In that case, or in the case of any other legitimate message for which the sender does not respond to the challenge (such as an automated message from a legitimate vendor), you will be able to find the message in the "Pending" folder on the Mailblocks server.
Messages for which challenges have been issued are held in the Pending folder until the sender responds; the user moves them to the inbox (at which time the sender is added to the address list); or they are deleted by the user or the system.
The amount of time messages are held in "limbo" in the Pending folder is configurable, but I believe the default is five days or so. They are then automatically deleted. When I first migrated to the system, I checked the Pending folder once a day or once every other day to make sure nothing legimate slipped through. Gradually, that became once a week, then once a month or so. After having used it for a year now, I rarely check it all.
I might add that even if a pesky spammer (or anyone else you don't wish to communicate with) does respond to the challenge and winds up in your address book, it's an easy matter to have future messages from them permanently blocked.