Simple Mail Transfer Protocol

The most standard mailing protocol

1. History Simple Mail Transfer Protocol  ( SMTP ) is an Internet standard for electronic mail (email) transmission . First defined by RFC 821in 1982 SMTP became widely used in the early 1980s. At the time, it was a complement to Unix to Unix Copy Program (UUCP) mail, which was better suited for handling email transfers between machines that were intermittently connected.

2. Introduction SMTP (Simple Mail Transfer Protocol) is a  TCP/IP   protocol  used in sending and receiving e-mail . Users typically use a program that uses SMTP for sending e-mail and either POP3 or IMAP for receiving e-mail.

3. SMTP Concept

4. Working SMTP is a simple ASCII protocol that is based on client-server model. After establishing the TCP connection, the sending machine, operating as the client, waits for the receiving machine, operating as the server, to talk first. The server starts by sending a line of text giving its identity and telling whether or not it is prepared to receive mail. If it is not, the client releases the connection and tries again later. If the server is willing to accept e-mail, the client announces whom the e-mail is coming from and destination, the server gives the client the go ahead to send the message. Then the client sends the message and the server acknowledges it.

5. Smtp commands The HELO command is used to initiate an SMTP session. When one host needs to establish an SMTP session with another host MAIL FROM: command is used primarily to send email addresses The RCPT TO: command tells the receiving host the email address of the message recipient

6. Smtp commands contd. DATA has the body of the mail The QUIT command is used to terminate an SMTP session The RSET command performs an SMTP reset, and then aborts the message that is currently being sent . EXPN includes the mailing list to be expanded

7. SMTP response codes Responses are 3 digit codes sent from server to client 220 : The SMTP service is ready 221 : SMTP is closing the transmission channel 250 : The command has been completed 354 : OK to transmit message 450 : Command can not be completed because the mailbox is busy

8. 451 : Command has been aborted because of an error 452 : Command has been aborted because the receiving host  is out of disk space 500 : Syntax error 550 : Specified mailbox is unavailable or does not exist 552 : Command was aborted because the recipient has exceeded  their storage quota 554 : The transaction has failed

9. Mail Transfer Phases Connection Establishment Mail transfer Connection termination

10. Connection Establishment An SMTP sender will attempt to set up a TCP connection with a target host when it has one or more mail message to deliver to that host. The following sequence occurs during connection setup:   1. The sender opens a TCP connection with the receiver. 2. Once the connection is established, the receiver identifies itself with '220 Service Ready". 3. The sender identifies itself with the HELO command. 4. The receiver accepts the sender's identification with "250 'OK". 5. If the mail service on the destination is not available, the destination host returns a "421 Service Not Available" reply in step 2 and the process is terminated.

11. Mail Transfer • Once the connection has been established, the SMTP sender may send one or more messages to the SMTP receiver. • There are three logical phases to the transfer of a message : 1. A MAIL command identifies the originator of the message. 2. One or more RCPT commands identify the recipients of this message. 3. A DATA command transfers the message text.

12. Connection termination The SMTP sender closes the connection in the following manner: 1. The sender sends a QUIT command and waits for a reply. 2. Sender initiates TCP close operation for the TCP connection. 3. The receiver initiates its TCP close after sending is reply to the QUIT command.

13. LATEST DEVELOPMENTS

14. Today’s email content needs to be lively, dynamic and time-dependent . A drop in click rates and falling conversion rates, just like everyone else who still sends the same email to everyone in their database

15. The latest development is real-time content in emails. Previously the content of an email was fixed at the time it was sent. Now the content of your mails can be updated in real-time, every single time the email is opened.

16. Examples Offer of the day Live data Limited time content Social content

17. Agile emails The most recent development stage is agile emails where the content is updated each time the email is opened. Combined with dynamic content and automisation , this creates fantastic and unlimited opportunities.

18. IDN International email  ( IDN email  or  Intl email ) is email that contains international characters (characters which do not exist in the  ASCII  character set), encoded as  UTF-8 , in the  email header  and in supporting mail transfer protocols. The most significant aspect of this is the allowance of  email addresses  (also known as email identities) in most of the world's writing systems, at both interface and transport levels.

19. IDN Contd. A Russian might wish to use  дерек@екзампил.ком  as their identifier but be forced to use a transcription such as  [email protected] International email, by contrast, uses  Unicode  characters encoded as  UTF-8  - allowing for the encoding the text of addresses in most of the world's writing systems

20. Usage All email servers use SMTP godaddy - 250 msgs per day gmail - 150 msgs a day B ut we can pay and create if we want to create full fledged email servers

21. Evolution Earlier using SMTP only text needed to be transferred. However nowadays, additional features such multimedia also Security and encryption features added

22. B oth provide a way to encrypt a communication channel between two computers (e.g. your computer and our server). TLS is the successor to SSL and the terms SSL and TLS are used interchangeably unless you're referring to a specific version of the protocol.

23. Versions STARTTLS is a way to take an existing insecure connection and upgrade it to a secure connection using SSL/TLS. Note that despite having TLS in the name, STARTTLS doesn't mean you have to use TLS, you can use SSL So the ordering of protocols in terms of oldest to newest is: SSL v2, SSL v3, TLS v1.0, TLS v1.1, TLS v1.2, TLS v1.3 (currently proposed).

24. Recently Google enabled smtp / tls - for security purposes. both parties need to support this. When you connect to an SSL/TLS encrypted port, or use STARTTLS to upgrade an existing connection, both sides will negotiate which protocol and which version to use based on what has been configured in the software and what each side supports. SSL/TLS is virtually universal these days

25. SSL v2 was deprecated and phased out many years ago due to security issues. SSL v3 is currently deprecated due to security issues, and support has or is being phased out by most services these days. Almost all software supports TLS v1.0. As at October 2016, support for TLS v1.1 and TLS v1.2 is also very good, though there are some exceptions, mostly with Internet Explorer.

26. To add security to some existing protocols (e.g. IMAP, POP, etc.), it was decided to just add SSL/TLS encryption as a layer underneath the existing protocol. To distinguish that software should talk the SSL/TLS encrypted version of the protocol rather than the plaintext one, a different port number was used for each protocol. So you have: IMAP uses port 143, but SSL/TLS encrypted IMAP uses port 993. POP uses port 110, but SSL/TLS encrypted POP uses port 995. SMTP uses port 25, but SSL/TLS encrypted SMTP uses port 465.

27. This has now basically become the de facto standard that everyone uses. IMAP SSL/TLS encrypted over port 993 or POP SSL/TLS encrypted over port 995. Many sites (including FastMail ) now disable plain IMAP (port 143) and plain POP (port 110) altogether so people must use an SSL/TLS encrypted connection.

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