Hi,
Generally all we are well known about mailing. The important protocol for mailing is SMTP. we see the details below

Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. Formally SMTP is defined in RFC 821 (STD 10) as amended by RFC 1123 (STD 3). The protocol used today is also known as ESMTP and defined in RFC 2821.

Description
SMTP is a relatively simple, text-based protocol, where one or more recipients of a message are specified (and in most cases verified to exist) and then the message text is transferred. It is quite easy to test an SMTP server using the telnet program (see below).
SMTP uses TCP port 25. To determine the SMTP server for a given domain name, the MX (Mail eXchange) DNS record is typically used, falling back to a simple A record in the case of no MX (not all MTAs (Mail Transfer Agents) support fallback). Some current mail transfer agents will also use SRV records, a more general form of MX, though these are not widely adopted.
SMTP is a "push" protocol that does not allow one to "pull" messages from a remote server on demand. To do this a mail client must use POP3 or IMAP. Another SMTP server can trigger a delivery in SMTP using ETRN.

History
SMTP developed out of Mail Box Protocol (ca. 1971), FTP Mail (ca. 1973),[1] and Mail Protocol.[2] The work continued throughout the 1970s, until the ARPANET converted into the modern Internet around 1980. Jon Postel then proposed a Mail Transfer Protocol in 1980 that began to remove the mail's reliance on FTP.[3] SMTP was published as RFC 821 in 1982, also by Jonathan Postel.
SMTP started becoming widely used in the early 1980s. At the time, it was a complement to UUCP (Unix to Unix CoPy) which was better suited to handle e-mail transfers between machines that were intermittently connected. SMTP, on the other hand, works best when both the sending and receiving machines are connected to the network all the time. Both use a store and forward mechanism and are examples of push technology.
The article about sender rewriting contains technical background info about the early SMTP history and source routing before RFC 1123.
Sendmail was one of the first (if not the first) mail transfer agent to implement SMTP. As of 2001 there are at least 50 programs that implement SMTP as a client (sender of messages) or a server (receiver of messages). Some other popular SMTP server programs include Postfix, qmail, Novell GroupWise, Exim, Novell NetMail and Microsoft Exchange Server.
Since this protocol started out as purely ASCII text-based, it did not deal well with binary files. Standards such as Multipurpose Internet Mail Extensions MIME were developed to encode binary files for transfer through SMTP. MTAs developed after sendmail also tended to be implemented 8-bit-clean, so that the alternate "just send eight" strategy could be used to transmit arbitrary data via SMTP. Non-8-bit-clean MTAs today tend to support the 8BITMIME extension, permitting binary files to be transmitted almost as easily as plain text.

Sample communications
After establishing a connection between the sender (the client) and the receiver (the server), the following is a legal SMTP session. In the following conversation, everything sent by the client is prefaced with C: and everything sent by the server is prefaced with S:. On most computer systems, a connection can be established using the telnet command on the client machine, for example.
telnet Example Web Page 25
which opens a TCP connection from the sending machine to the MTA listening on port 25 on host Example Web Page.

S: 220 Example Web Page ESMTP Postfix
C: HELO mydomain.com
S: 250 Hello mydomain.com
C: MAIL FROM:<sender@mydomain.com>
S: 250 Ok
C: RCPT TO:<friend@example.com>
S: 250 Ok
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: Subject: test message
C: From: sender@mydomain.com
C: To: friend@example.com
C:
C: Hello,
C: This is a test.
C: Goodbye.
C: .
S: 250 Ok: queued as 12345
C: QUIT
S: 221 Bye

Although optional and not shown above, nearly all clients ask the server which SMTP extensions the server supports, by using the EHLO greeting to invoke Extended SMTP (ESMTP). These clients use HELO only if the server does not respond to EHLO.
Contemporary clients will use the ESMTP extension keyword SIZE to inquire of the server the maximum message size that will be accepted. Older clients and servers will try to transfer huge messages that will be rejected after wasting the network resources, including a lot of connect time to dialup ISPs that are paid by the minute.
For the edit planning of giant files or sending with older clients, users can manually determine in advance the maximum size accepted by ESMTP servers. The user telnets as above, but substitutes "EHLO mydomain.com" for the HELO command line.

S: 220-serverdomain.com ESMTP {postfix version and date}
S: 220 NO UCE. {etc., terms of service}
C: EHLO mydomain.com
S: 250-serverdomain.com Hello mydomain.com [127.0.0.1]
S: 250-SIZE 14680064
S: 250-PIPELINING
S: 250 HELP
This serverdomain.com declares that it will accept a fixed maximum message size no larger than 14,680,064 octets (8-bit bytes). Depending on the server's actual resource usage, it may be currently unable to accept a message this large.
In the simplest case, an ESMTP server will declare a maximum SIZE with only the EHLO user interaction. If no number appears after the SIZE keyword, or if the current message limit must be exactly determined, the user can further interact by simulating the ESMTP header of a message with an estimated size. See External Link RFC 1870 below.
Security and spamming
One of the limitations of the original SMTP is that it has no facility for authentication of senders. Therefore the SMTP-AUTH extension was defined. However, the impracticalities of widespread SMTP-AUTH implementation and management means that E-mail spamming is not and cannot be addressed by it.
Modifying SMTP extensively, or replacing it completely, is not believed to be practical, due to the network effects of the huge installed base of SMTP. Internet Mail 2000 is one such proposal for replacement.
Spam is enabled by several factors, including vendors implementing broken MTAs (that do not adhere to standards, and therefore make it difficult for other MTAs to enforce standards), security vulnerabilities within the operating system (often exacerbated by always-on broadband connections) that allow spammers to remotely control end-user PCs and cause them to send spam, and a regrettable lack of "intelligence" in many MTAs still a major problem.
There are a number of proposals for sideband protocols that will assist SMTP operation. The Anti-Spam Research Group (ASRG) of the Internet Research Task Force (IRTF) is working on a number of E-mail authentication and other proposals for providing simple source authentication that is flexible, lightweight, and scalable. Recent Internet Engineering Task Force (IETF) activities include MARID (2004) leading to two approved IETF experiments in 2005, and DomainKeys Identified Mail in 2006.

__________________
With,
J. Jeyaseelan
Everything Possible