While spam may seem like a minor issue on a small scale, it is a significant problem on the Internet. A system hosting hundreds or thousands of users each receiving dozens or hundreds of unwanted messages every day can have substantial difficulties dealing with the onslaught. There is a real cost to the victims of spam. It unfairly uses the bandwidth and disk space of its recipients and their providers.
Other costs brought on by spam include technical support personnel time, when technicians or administrators must help users clean up flooded mailboxes. Sometimes the volume of spam can even make a system unusable for its intended purpose (clogging bandwidth or filling disk space). In such a case the effects of spam are no different from those of a denial-of-service attack. Even in less drastic circumstances, spam interferes with legitimate uses of email. Important messages can easily be overlooked in a flood of spam or mistakenly deleted when littered mailboxes are cleaned up.
A significant issue with spam is dealing with messages addressed to nonexistent users. Some mail systems recognize that a destination address is bogus and can reject mail before it is accepted; other systems must receive the mail first and then bounce it as undeliverable. The volume of bounces can easily clog a queue and interfere with the delivery of legitimate messages. Since the return addresses often don't really exist, the bounces cannot be delivered and sit in the queue undergoing many redelivery attempts until they expire.
Another spamming trick is to use a legitimate return address that belongs to an innocent third party. The target or relay systems that receive the spam send bounce messages to the supposed sender, helpfully letting that person know that the recipient does not exist. In this case, thousands or millions of bounce messages will be delivered to the unfortunate victim in a phenomenon referred to as backscatter. This victim isn't involved in any way in the original delivery of the spam. In most cases, the only solution for these completely innocent bystanders is to abandon the victimized address and start using a new one.