Unsolicited Bulk Email (UBE), also referred to as Unsolicited Commercial Email (UCE), is commonly called spam. Spamming is the practice of sending mass mailings to large numbers of people who have had no prior relationship with the sender and who didn’t ask to receive such mail. Spam exists because it’s so cheap to send. The incremental cost of adding even hundreds of thousands of recipients to a mailing is relatively small, so spammers target as many email addresses as they possibly can. This chapter looks at the problem of spam and the tools Postfix provides to help limit the consequences.