SMTP(Simple Mail Transfer Protocol) is a push protocol and is used to send the mail whereas POP (post office protocol) or IMAP (internet message access protocol) are used to retrieve those emails at the receivers or client-side. The SMTP server on Linux is very fast, reliable and secure. Also, it supports POP3, IMAP and webmail access. Linux systems in a network can use the SMTP server to send alerts notifications. The mail transfer agent is an application use SMTP to transmit Email over the network. We have some of the most popularly used open-source Mail transfer agents like POSTFIX, SENDMAIL, EXIM, QMAIL, MUTT, ALPINE etc.. Each agent has its own advantages and disadvantages. You can review your system and can install the required one as per need. Postfix is a free and open-source mail MTA (Mail Transfer Agent). This application used to send and receive the email. It is responsible for routing and delivering electronic mail. This is a cross-platform and most popular system.