POP before SMTP or SMTP after POP is a method of

authentication Authentication (from ''authentikos'', "real, genuine", from αὐθέντης ''authentes'', "author") is the act of proving an assertion, such as the identity of a computer system user. In contrast with identification, the act of indicati ...

used by

mail server Within the Internet email system, a message transfer agent (MTA), or mail transfer agent, or mail relay is software that transfers electronic mail messages from one computer to another using SMTP. The terms mail server, mail exchanger, and MX host ...

software which helps allow users the option to send e-mail from any location, as long as they can demonstrably also fetch their mail from the same place. The POP before SMTP approach has been superseded by

SMTP Authentication SMTP Authentication, often abbreviated SMTP AUTH, is an extension of the Simple Mail Transfer Protocol (SMTP) whereby a client may log in using any authentication mechanism supported by the server. It is mainly used by submission servers, where au ...

. Technically, users are allowed to use

SMTP The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typical ...

from an

IP address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...

as long as they have previously made a successful

login In computer security, logging in (or logging on, signing in, or signing on) is the process by which an individual gains access to a computer system by identifying and authenticating themselves. The user credentials are typically some form ...

into the POP service at the same mail hosting provider, from the same address, within a predefined timeout period. The main advantage of this process is that it was generally transparent to the average user who will be connecting with an

email client An email client, email reader or, more formally, message user agent (MUA) or mail user agent is a computer program used to access and manage a user's email. A web application which provides message management, composition, and reception functio ...

, which almost always attempted to fetch new mail before sending new mail. The disadvantages include a potentially complex setup for the mail hosting provider (requiring some sort of communication channel between the POP service and the SMTP service) and uncertainty as to how much time users will take to connect via SMTP (to send mail) after connecting to POP. Those users not handled by this method need to resort to other authorization methods. Also, in cases where users come from externally controlled dynamically assigned addresses, the SMTP server must be careful about not giving too much leeway when allowing unauthorized connections, because of a possibility of

race condition A race condition or race hazard is the condition of an electronics, software, or other system where the system's substantive behavior is dependent on the sequence or timing of other uncontrollable events. It becomes a bug when one or more of t ...

s leaving an

open mail relay An open mail relay is a Simple Mail Transfer Protocol (SMTP) server configured in such a way that it allows anyone on the Internet to send e-mail through it, not just mail destined to or originating from known users. This used to be the default con ...

unintentionally exposed.

See also