| Simple Mail Transfer Protocol (SMTP) is the de facto standard/protocol for email transmission over mutually TCP accessible (sending and receiving) hosts on the Internet as shown in the diagram below, mutually-TCP-accessible hosts on a firewall isolated TCP/IP intranet. SMTP can also transmit email on hosts on a LAN or WAN environment utilizing a non-TCP transport-level protocol. Using SMTP, a process can transfer an email message to another process on the same network or to some other network via a relay or gateway process accessible to both networks (sender/receiver). |
| An email message can be sent to one or many recipients with qualified email addresses. Each email address uniquely identifies the mailbox address of a user on an Internet domain, such as webmaster, username1, username2, etc. on the domain "kayshav.com". The email address format is "username@domain_name". Other domain extensions are listed in reference [3]. |
| Example: webmaster@kayshav.com |
| Initially developed in the 1980s, this protocol was designed for email transmission of ASCII text files and later on standards such as MIME were developed to encode and transfer binary files as well. The SMTP uses the TCP port 25. The SMTP works well when both the sending and receiving mail servers are connected to the network all the time. SMTP is independent of the particular transmission subsystem and requires only a reliable ordered data stream chennel. SMTP ensures successful delivery of an email message by the use of commands and numeric codes that are transparent to the sender/receiver. When the destination address is incorrect, the SMTP is designed to send a "undeliverable mail" notification to the sender. The domain name, email address and the SMTP commands are not case sensitive. SMTP is one of the TCP/IP application layer core protocols in the Internet Protocol Suite. |
| In SMTP, a message can travel from source to destination in a single pass or hop through several intermediary relay or gateway hosts on its route from sender to the ultimate recipient. The Mail eXchanger mechanisms of the DNS will identify the order of hops for the message, in its route to the destination |
| SMTP Commands |
| DATA - DATA |
| EHLO - Extended HELLO |
| EXPN - EXPAND |
| HELO - HELLO |
| HELP - HELP |
| MAIL - MAIL |
| NOOP - NOOP |
| QUIT - QUIT |
| RCPT - RECIPIENT |
| RSET - RESET |
| VRFY - VERIFY |
| SMTP Reply Codes By Function Groups (listed by code number) | |
| 211 | System status, or system help reply |
| 214 | Help message |
| 220 | <domain> Service ready |
| 221 | <domain> Service closing transmission channel |
| 250 | Requested mail action okay, completed |
| 251 | User not local; will forward to <forward-path> |
| 354 | Start mail input; end with <CRLF>.<CRLF> |
| 421 | <domain> Service not available, closing transmission channel |
| 450 | Requested mail action not taken: mailbox unavailable (busy) |
| 451 | Requested action aborted: error in processing |
| 452 | Requested action not taken: insufficient system storage |
| 500 | Syntax error, command unrecognized |
| 501 | Syntax error in parameters or arguments |
| 502 | Command not implemented |
| 503 | Bad sequence of commands |
| 504 | Command parameter not implemented |
| 521 | Host does not accept mail |
| 550 | Requested action not taken: mailbox unavailable |
| 551 | User not local; please try <forward-path> |
| 552 | Requested mail action aborted: exceeded storage allocation |
| 553 | Requested action not taken: mailbox name not allowed (mailbox syntax incorrect) |
| 554 | Transaction failed |
| 556 | Domain does not accept mail |
| Many applications, database engines, IoT, cloud services, etc. have integrated SMTP (Oracle) to enable emails from processes to automate internal workflow. They are also used in error handling and warning. Email has become the way to communicate reservations, billing/payment confirmation and in social media for communication. |
| Reference information and links: |
| 1. J. Klensin, Simple Mail Transfer Protocol, RFC-2821, AT&T Laboratories, April 2001.. |
| 2. Jonathan B. Postel, Simple Mail Transfer Protocol, RFC-821, USC/Information Sciences Institute, August 1982. |
| 3. Generic Top Level Domain Extensions gTLD |
| 4. Latest SMTP updates (521 and 556 Reply Codes), RFC-7504, June 2015. |