Simple Mail Transfer Protocol (SMTP)
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SMTP uses straight forward, simple, efficient and relatively to transfer mail between hosts, It was first defined in RFC 821. SMTP has two rules sender acts as client it establish a TCP connection with the receiver , which act like a server. SMTP uses a simple text-based technique in which one or more recipients of mail are defined.
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SMTP Model
SMTP Required reliable ordered data stream channel to transfer mail usees the following model of communications[1]:
- when the mail request is initiated, the SMTP sender establish a two way transmission channel connect the sender and the receiver.
- The reciver can be intermediate or final destination for the mail.
- Generating a SMTP commands by the sender and send to the receiver.
- The receiver then send back a reply as a response to the commands.
- The response can be an Okay response if the receiver can accept the mail.
- The sender responds by sending a RCPT command to identify the mail recipient.
- The receiver may rejects the recipients, but cannot reject the whole transaction.
SMTP sender and receiver may negotiate the several recipients, and then SMTP send email data, if the receiver is successfully processed and accepted, then the mail data send an Oay reply[1].
Time stamps and message ID
SMTP added time when email is received by your computer , this feature also include store tracking data of all hosts that relayed the message. Usually SMTP Message Transfer Agent is used to show the time stamp at the start of message, as well as the sender and each time that an item relayed this time stamp shows the ID for the host send the message, the ID that received the message, and the time that the message was received [2], the important of this information become from the fact that most of the time stamp and host identity enhance the chance for the receiver to distinguish between email sent from a trusted destination or crackers. Also it provide an important information if a problem in mail delivery happened. Time stamps not always a reliable information source as computer clocks are sometimes inaccurately set , or the information sequence is not make since, adding the time stamp is optional by the recipient user agent.
Bouncing email
It is not always true that every email will be delivered to its destination , this happened if the sender provide incorrect recipient ID, in this case the email send back to originator with a message indicate the reason of why the mail not delivered , the returned email called Bounced email.
Security
Many critical security issues related to emails and emails agents, one of the most used agents is sendmail, it is complicated and big program that have many functions including mail translation and mail alias names. Send mail mainly use SMTP which run NVT telnet , as known telnet has so many security issues by using open text format to send data, NVT telnet use port 25 , if any body try to break into a computer this is a great security hole to do so, other limitation on original SMTP is it has no way to authenticate senders. some easy solutions are used by using more secure and efficient mail agents written with some kind of mail encryption and spam filter, or even have an assistant virus scanner to scan emails before has open them. call back verification also can be used as validate email address this mostly used as ant spam measure.
Some of SMTP commands
SMTP commands example
HELO : Identify the sender MAIL FORM : Start mail transaction and identify mail originator RCOT TO : Identify an individual recipient DATA : Sender is ready to transmit a series of line of text VERFY : Ask for name identification from the reciver TURN : Ask partiner to switch roles and become a sender SOML : Send email if reciver is logged in, deliver direct to the terminal
See Also
References
- [2] TCP/IP architecture , protocols, and implementation with IPv6 and IP security. second edition, McGraw-Hill , Dr. Sidnie Feit. 1996.
External Links
- SMTP in Wikipedia
- [2] Extended SMTP Wikipedia
- SMTP Commands - SMTP complete command list
- Xming - A free X server for Windows
- Cygwin - A linux like environment for Windows that contains an SSH package and an X server
- OpenSSH - Open source implementation of SSH
- MidpSSH - An SSH client for mobile devices
