The Post Office Protocol version 3 (POP3) is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection.
A constant connection to the mail server is not necessary when using POP3. The connection to the server is build up when needed and closed afterwards.
POP3 is limited in its functions and it only allows listing, retrieving and deleting mail on the mail server. Some more features like hierarchical mailbox direct on the mail server, access to several mailboxes during a session, selection of e-mails etc. could only be used with IMAP.
IMAP (Interactive Mail Access Protocol) is another protocol for retrieving e-mail from the mail server. Here normally the mail will be encrypted with SSL. Instead of POP3 IMAP has some more features. So it is important to know the main principle.
Using POP3 the e-mails are retrieved via internet from the mail server, normally deleted and saved on your computer.
Using IMAP the mails are leaved on the server, read and managed there. If necessary, you also can download them on your own computer - and back. On the server the mail only are deleted if you delete them with your mail-client.
htt/Advantage of IMAP
IMAP is very interesting in case of using more than one computer for reading e-mails (e.g.: your computer at work, your computer at home or your laptop). You can sort and file your mails from multiple computers. You can see old e-mail, for instand in the sent folder, at all computers.
You can read the subject of the mail before downloading. Mail with no interest can then be deleted on the server. By that you can save time of download - important for slow connections, especially if some wit sent a mega-mail.
E-mails can be sent directly to a subfolder. If there is e.g. a mail address firstname.lastname@example.org with the subfolder test you can send an e-mail to email@example.com. It is copied directly into the subfolder test by the mail server. This feature is particularly interesting in case of archivating mails of a mailing-list membership.
Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks. SMTP is a relatively simple, text-based protocol, in which one or more recipients of a message are specified (and in most cases verified to exist) along with the message text and possibly other encoded objects. The message is then transferred to a remote server using a series of queries and responses between the client and server. An e-mail client knows the outgoing mail SMTP server from its configuration. The SMTP client initiates a TCP connection to server\'s port 25 (unless overridden by configuration).
Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide security and data integrity for communications over TCP/IP networks such as the Internet. Several versions of the protocols are in wide-spread use in applications like web browsing, electronic mail, Internet faxing, instant messaging and voice-over-IP (VoIP).