SMTP is based on end-to-end delivery; an SMTP client will contact the destination host's SMTP server directly to deliver the mail. It will keep the mail item being transmitted until it has been successfully copied to the recipient's SMTP. This is different from the store- and-forward principle that is common in many mailing systems, where the mail item may pass through a number of intermediate hosts in the same network on its way to the destination and where successful transm-ission from the sender only indicates that the mail item has reached the first intermediate hop.

The SMTP design is based on the model of communication. As a result of a user mail request, the sender-SMTP establishes a two-way connection with a receiver -SMTP. The receiver-SMTP can be either the ultimate destination or an interme-diate (mail gateway). The sender-SMTP will generate commands which are replied to by the receiver-SMTP.

Is your ISP blocking port 25?

Port 25 is the standard port used by smtp. Many ISPs have started to block that portoutside of their own system. Why? Spammers are under fire from every direction, if a ISP lets spam go out thru their doors, it is not good for them or their clients, the spam can clog their own systems and some people have tried to sue. In order to protect themselves and their clients they have started blocking port 25, which keeps people connected thru them from connecting to any SMTP server but their own. If you have a site that is with a web host, not your ISP, chances are that you have never been able to send email using your web host's smtp server. If that is the case, your ISP have probably started blocking port 25.