SMTP stands for Simple Mail Transfer Protocol. It is used when email is delivered from an email client, such as Outlook Express, to an email server or when email is delivered from one email server to another. Simple Mail Transfer Protocol uses port 25.
Your e-mail client (such as Outlook, Eudora, or Mac OS X Mail) uses this protocol to send a message to the mail server, and the mail server uses Simple Mail Transfer Protocol to relay that message to the correct receiving mail server.
Most server names are written in the form “smtp.domain.com” or “mail.domain.com”: for instance, a Gmail account will refer to smtp.gmail.com. But that’s not a unified rule, so you should pay attention and get the right Simple Mail Transfer Protocol parameters.
There are several ways to know your Simple Mail Transfer Protocol server name.
Remember anyway that only a professional Simple Mail Transfer Protocol server will guarantee that all your emails will be correctly delivered. Generally speaking, free outgoing servers like the ones associated to Gmail or Hotmail are not designed for mass email sending.