4.1.1 Unsolicited Commercial E-Mail (Spam)
A significant business has evolved around the sending of unsolicited commercial e-mail (colloquially referred as spam). Unlike physical bulk mail, with its built-in restrictions on labeling and cost, the general structure of SMTP allows anonymous sources to send e-mail messages virtually without restriction. Attempts are being made to reduce the volume of spam that makes it to a person's mailbox, but care has to be taken to not affect legitimate senders.
Part of the success of this industry is the fact that people impute importance to unverifiable things. For example, the purported sender of an e-mail message (considering most e-mail messages are not digitally signed) is commonly used by people to attach importance and priority. If the e-mail message appears to come from a person's boss, there is a higher probability that the employee would act on the message. In this case, care needs to be taken when receiving e-mail over unauthenticated transports. Even if the routing address of the sender matches a valid employee or contact (3), it is better if clients and servers preserve the external routing address on the Message object, because replacing it with its address book equivalent could impute elevated importance to the content.