Short for Simple Mail Transfer Protocol, a protocol for sending e-mail messages between servers. Most e-mail systems that send mail over the Internet use SMTP to send messages from one server to another; the messages can then be retrieved with an e-mail client using either POP or IMAP. In addition, SMTP is generally used to send messages from a mail client to a mail server. This is why you need to specify both the POP or IMAP server and the SMTP server when you configure your e-mail application.
Pop, with this type of email account you only have one folder, the Inbox folder. When you open your mailbox, new mail is moved from the host server and saved on your computer. If you want to be able to see your old mail messages, you have to go back to the computer where you last opened your mail.
With this type of account you do not have to stay logged on to the Internet. You can log on when you want to receive and send new messages. Once your new messages have been downloaded to your computer you can log off to read them. This option is good when you connect with your modem to DAS and are charged for your connection or you have an older computer.
Imap, using this protocol, all your mail stays on the server in multiple folders, some of which you have created. This enables you to connect to any computer and see all your mail and mail folders. In general, IMAP is great if you have a dedicated connection to the Internet or you like to check your mail from various locations.
That's my knowledge about the email server. Hope that might help you,