Smtp
On UNIX-based systems, Sendmail and Postfix are mostly used as SMTP servers for e-mails. Postfix has grown in popularity in recent years because it is easier to configure and it is more modular. In the commercial version, Sendmail already includes a POP3 server. Finally, Microsoft Exchange also offers an SMTP server and can be configured to support POP3. SMTP is usually implemented in such a way that it operates over port 25 on the Internet....