What is SPAM?
SPAM, also known as unsolicited email, has many definitions to many people. Some people consider it only unsolicited commerical email - the kind of email that is trying to sell you something. Others consider it any email that you did not wish to see. MAPS does not really attempt to define SPAM rather it simply classifies email as either permitted or not. Initially all email is considered not permitted. It is only when others register for permission to email you that MAPS considers email as wanted.
Initially all email will be returned to the sender with a message that describes how to register for permission to email you. Returned email is saved for up to 30 days (configurable) so that if the sender decides to register their previous email(s) will be delivered. If they register then all previous emails will be delivered and they will be added to your white list. Future emails from them will be delivered instead of returned.
Typically spammers are really robots or scripts that send thousands or millions of emails to address lists. They don't read returned messages so they will not register for permission to email you. Occasionally a spammer, usually a small operation, will read the returned message and may register. If this happens then you can easily blacklist that spammer and not be bothered by them again. As a MAPS user myself who receives probably more SPAM than you will ever see I can say that perhaps one to two real spammers will register every other month. So you can easily deal with such annoyances.
Because spammers often use invalid email addresses or email address that quickly fill up with "Please don't bother me" return messages, often a MAPS register message will be returned by a mailer daemon telling you that the spammer's email address doesn't exist or is full. You don't want to be bothered with such return messages so MAPS seeds your null list with entries to prevent this. If you receive emails from such mailer daemons and do not wish to receive them simply null list them. The null list is also good for other annoying email that you receive that you'd rather not be bothered with. For example, you might receive a newsletter sort of email from a company you normally wish to deal with but are not really interested in their newsletters. Perhaps the newsletters are send from an address of newsletters@<company I care about>.com where other email might come from support@<company I care about>.com. In that case you can safely null list newsletters@<company I care about>.com. For exmaple, I null list email@example.com because I do not wish to receive those information emails from Netflix.com about shipments.
Your black list is similar to your null list except instead of merely discarding the email, a return message is sent to the sender saying that they are blacklisted. This is good for people who you wish to make sure know that you are consciously ignoring them.