Mail Server, POP3, IMAP and SMTP

Mail Server, POP3, IMAP and SMTP are related to e-mail and e-mail delivery system. A mail server is a network application located on a server on the Internet whose function is similar to postal mail but in this case the mail (sometimes called messages) are circulated, they do so through data transmission networks and unlike postal mail, hereby can only send attached files of any length to the information in electronic form.

Web Mail Server

A special form of mail server is one that is accessed via the Web using http protocol. It is not really a server, but a mail client that runs on a web server. Through this client one can access the mail server without installing an email client on the local computer.

In this type of server, the file sender or recipient data can be accessed without requiring a local client. On the same server to access programs are integrated to the same post. Typical examples of this service are: hotmail.com, yahoo.com, gmail.com etc.

Functions of Mail Server

Most mail server work together some of these multiple services:

MTA (Mail Transfer Agent)

This service provides complete transport, for example by e-mail program to MTA or MTA to MTA. This is usually the SMTP protocol used, in rare cases UUCP or X.400. The MTA stores messages not permanent but only temporarily, until he has passed successfully.

Mail filters

On some mail servers are blacklists, spam filters, anti-virus programs and other filters used.

MDA (Mail Delivery Agent)

This service stores the e-mails to the various e-mail inboxes. In some cases, for better communication between the MTA and MDA, the Local Mail Transfer Protocol (LMTP) is used.

MRA (Mail Retrieval Agent)

This service allows the stored e-mails from the mail server from an e-mail program (MUA) can be downloaded. Most protocols are POP3 or IMAP support.

Protocols / ports associated with Mail Server Software

In the operation of a mail server following protocols and ports are used:

• SMTP / ESMTP protocol is that, with the e-mails are sent. (Port 25)
• POP3 is used to retrieve mail from a mail server. (Port 110)
• IMAP is used to access mailboxes that are on mail servers. (Port 143)
• SMTPS (port 465), pop3s (port 995) and IMAPS (port 993), the SSL / TLS secure versions of SMTP, POP3 and IMAP.
• APOP and KPOP POP3 extensions are to avoid the unencrypted transmission of login information.
  Message Submission Agent (MSA). (Port 587)
• UUCP is a protocol to be, with the e-mails sent and collected during the next connection. It has now lost much of its significance.
• MIME encoded e-mails and determine their structure.
• Email Address Verification Protocol (Draft) – test for the existence of an e-mail accounts (draft).