Simple Mail Transfer Protocol
The SMTP is designed to transfer email messages reliably and efficiently, again without regards to the particular computers or operating systems encountered along the way. It does this by setting up a channel between the initial sender and a receiver, which can be either the ultimate destination or some waypoint. Once the transmission channel is established, the mail sender issues a MAIL command, which identifies the sender and states that there is traffic to send. If the mail receiver can accept mail, it responds with an OK reply. The mail sender then sends a RCPT command identifying the mail recipient. If the mail receiver can accept mail for that recipient, it responds with an OK reply. If not, it responds with a reply rejecting that recipient (but not the whole mail transaction).
The mail sender and mail receiver may negotiate with several recipients. When the recipients have been negotiated, the sender sends the mail data. If the SMTP receiver successfully processes the mail data, it responds with an OK reply.
In the case that mail is sent to an intermediary stop, or waypoint, the process is repeated. If the mail receiver is the intended destination, the message is forwarded to a mailbox for storage until the recipient calls for it with her mail client.
Mail that can't be delivered because of incorrect or invalid addresses are returned with a note from whichever mail server determined the problem, stating that delivery was impossible.
The SMTP system works so well that email has become an important means of doing business. This same reliability, however, is its undoing. Email is normally transmitted in the clear, which means that a host that pretends to be an email relay can access all email that passes through it; mail could then be copied or modified. When an attacker suspects that a user or administrator is getting suspicious, it is relatively easy to disconnect the relay and lay low. The flow of message receipts and returns may be delayed but will likely not be disrupted because of the self healing nature of the robust SMTP protocol.
Further, it is very easy to create an email message that looks as if it was sent from someone other than the true sender. This can create problems in its own right (for example, a university student notifies everyone in a class that a certain test has been cancelled, and the message appears to emanate from the professor's computer). This also makes it easy to formulate an attack that sends tens of thousands of emails out to various addresses on the Internet, valid or not, using the spoofed return address of someone you wish to annoy or attack. As the emails bounce off the bad recipient addresses, your target will get a flood of annoying messages saying that the address is no longer valid. A few of the addresses will be valid, so your victim may get a couple of irate responses from legitimate but uninterested recipients as well.