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A simple mail transfer protocol (SMTP) service is used when moving email between a client program and an email server, or from one server to another. During most SMTP transactions, identifying information about the sender of the message is included along with the message itself. By using an anonymous SMTP service, the sender's identity remains hidden, along with any other potentially identifiable characteristics of a message.
Computers connected to the Internet are assigned an Internet Protocol (IP) address by their network service provider. This is a unique identifying number typically expressed in a numerical format such as 123.456.78.90. When sending information across the Internet, the IP address is normally included with the transmission, much as a return address is required when sending a letter through the post office. Under ordinary circumstances, the IP address can be used to determine the location and identity of the sending computer, but not with an anonymous SMTP service.
An anonymous SMTP service blocks inclusion of the IP address in one of two ways. It can block the IP in its entirety, providing no "return address" whatsoever after sending a message; this is known as IP blocking. In the alternative, it provides the recipient of the message with a fake IP address, hiding the sender's real identity; this is known as IP masking. Anonymous SMTP services tend to use one method or the other, so users should fully understand each method's particular advantages and disadvantages.
Using an anonymous SMTP service which uses IP blocking is possibly the most reliable method of concealing an IP address, because it provides the recipient with absolutely no information to trace back. The downside to this is that by blocking the IP address completely, the message recipient immediately knows that the sender has made a deliberate effort to conceal their identity. This can cause the recipient to delete the message without even reading it, believing it might contain a virus or other type of malicious program.
IP masking is the flip side of the same coin. By providing a false "masked" IP address, the receiver will not necessarily realize the message has been sent from an anonymous source. If the recipient tries to trace the IP, he or she is likely to discover that it supposedly originated out of the country or from some other implausible location.