POP before SMTP
From Wikipedia, the free encyclopedia
POP before SMTP or SMTP after POP is a method of authorization used by mail server 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.
Technically, users are allowed to use SMTP from the same IP address as long as they have previously made a successful login into the POP3 service at the same server.
The main advantage of this process is that it's generally transparent to the average user who will be connecting with an email client, which will almost always make a connection to fetch new mail before sending new mail.
The disadvantages include a potentially complex setup (requiring some sort of communication between the POP service and the SMTP service) and uncertainty as to how much time will all users take to connect via SMTP (to send mail) after connecting to POP3, and will they indeed all do that. Those users not handled by this method need to resort to other authorization methods. Also, in cases where users come from externally controlled dial-up addresses (more specifically, all dynamically assigned IP addresses), the SMTP server must be careful about not giving too much leeway when allowing unauthorized connections, because of a possibility of race conditions leaving an open mail relay unintentionally exposed.
