ESTMP: It is a protocol for sending e-mail messages between servers. Mostly e-mail systems that send mail over the Internet use SMTP to send messages from one server to another; then the messages can be retrieved along with an e-mail client by using either POP or IMAP. Additionally, generally SMTP is used to send messages from mail client to another mail server. This is why you require specifying both the POP and IMAP server and the SMTP server while you configure your e-mail application.