SMTP Commands
SMTP commands are the commands that are used by a client computer to communicate with an SMTP (Simple Mail Transfer Protocol) server.
Before elaborating on SMTP commands, it is imperative to understand what an SMTP server is. An SMTP server refers to that application which serves the main purpose of receiving, sending and/or relaying outgoing mail between email senders and receivers. An SMTP server is similar to any other server with regard to its function of processing data in order to send it to another server but it is more specific in this regard as it processes specifically the data that is related to an email that is being sent, received or relayed. In the absence of an SMTP server, an email that has been sent, won’t arrive at its intended destination. Moreover, it is the SMTP server which corroborates that the outgoing email has originated from an active account. This lets it serve the purpose of being the first safety check when it comes to ensuring the protection of an inbox against illicit emails. In the event that an email fails to get delivered, it sends that email back to the sender. This is very beneficial as it ensures that the sender has the information that he doesn’t have the right email address or that the recipient sever is blocking his email.
Returning to our main topic regarding SMTP commands, these are sent by the client to the server. Every SMTP command is formed by a keyword that is followed by 0 (zero) or some argument. This implies that there are certain keywords which are devoid of any argument. This command has the following format, keywords: argument(s).
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Basic SMTP Commands
Returning to our main topic that has to do with SMTP commands, basic SMTP commands are those that are supported by all SMTP servers. Each of these commands is used between two servers and uses a normal communication sequence. SMTP protocol is followed in order to ensure the delivery of emails.
Extended SMTP commands (ESMTP commands) are supported by many SMTP servers, in order to ensure more flexibility and implement additional features. SMTP service extension is another term that is used for such an ESMTP command. These are not included in the information that is to follow. Information regarding all the basic SMTP commands, which the SMTP protocol has specified, is provided in the following paragraphs. These are mentioned in no particular order.
HELO- This SMTP command’s usage is mandatory and it provides the identity of the sender server (the host name). This command is sent by the client to the SMTP server in order to identify itself. It starts the SMTP conversation. The SMTP client’s IP address or domain name is sent in the form of an argument along with this command. A domain name that can be used as an argument with the HELO command, has to be an FQDN i.e. a fully qualified domain name.
MAIL FROM- The email address of the sender is specified by it and its usage is mandatory. Hence, the origin of the mail is specified by it. It signals the SMTP server the start of a new mail transaction. It ensures the reset of all the buffers and state tables of the server. This is usually the first command that is sent post the identification and login process. The source email address is stated by the sender in the “Form” field and then the email transfer starts. The server replies with a 250 OK reply code in the event that the sender’s email address is accepted.
DATA- The start of the mail is specified by it and its usage is mandatory. This command ensures the beginning of the process of transfer of the email content (body text, attachments etc.).
Once the client sends this command to the server, a 354 reply code is sent by the server in response. It implies that permission has been granted to begin the transmission. Post this, the content can be transferred to the server. A single dot (“.”) needs to be sent in a line by itself, once all the content of the message has been sent. The SMTP server sends a 250 reply code as a response, on accepting the message for delivery.
RCPT TO (Recipient To)- The recipient’s email address is specified by it. Its usage is mandatory as well. If there are multiple recipients of an email, this command gets repeated multiple times.
RSET (Reset)- This command, when sent to the mail server, terminates the current mail transaction. However, the TCP (Transmission Control Protocol) connection remains open. Every information that has to do with the email data, the sender as well as the recipient gets removed. Additionally, the state tables and the buffers are cleared. Its usage is highly advised.
NOOP (No Operation)- As is evident from the term itself, it refers to “no operation”. Its usage is highly recommended. This command has no other function except eliciting an “OK” reply from the receiver. It serves the purpose of confirming that the server is connected as well as able to communicate with the client.
VRFY (Verify)- This command is used to ascertain the username. Its usage is also highly recommended. It serves the function of ascertaining from the server that a particular username or email address is in existence. Some email servers ignore this basic SMTP command because of security issues. Some reply will be sent by those servers that ignore this command but the information that had been asked by the client won’t be sent.
QUIT- It ends the SMTP conversation and closes the TCP connection. Its usage is mandatory. The server is asked to close the connection by using this command. The server responds with a 221 code if the connection can be closed and terminates the session.
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