In Simple Mail Transfer Protocol( SMTP), the communication between mail client and mail server is supported by special commands. The commands are used for the identification of the mail server towards the mail client, furthermore for the sending of the message, the entry of the recipient address and the confirmation of the recipient address by the mail server.
In addition, the SMTP commands are used for the extension of the e-mail address to a group address, for the actual transmission of the message and for a possible interruption or confirmation of the transaction. Each command must be terminated by a control character and for each executed command the SMTP client receives a response from the SMTP server.
The SMTP commands are listed in RFC 2821 and include in detail: HELO is the SMTP command with which the mail client identifies itself to the SMTP server. The identification is limited to the complete domain name. The extension of the simple HELO command is EHLO (Extended Helo) where keywords are transmitted to the mail server. If the SMTP server supports EHLO extension, then it sends a response to the SMTP client.
The actual message transmissionis done with the MAIL command. As soon as the SMTP command MAIL is issued, the message is sent. If it cannot be delivered, the client is notified of the delivery problem. A single recipient message is entered with RCPT (Recipient). If multiple recipient addresses are to be entered, each is provided with an RCPT command. If the client wants a confirmation of the recipient address by the mail server, this is triggered with VRFY (Verify). This command is not supported by some providers. If the address entered is an address from a mailing list, then the server can expand the address: EXPN.
The transmission of the message is initiated from the client with DATA and then receives a response from the SMTP server. For the interruption of a transmission, stands the RSET. In this case, already transmitted addresses and messages are deleted. QUIT is a positive confirmation and is used to end the communication, and HELP is a help function for the SMTP client.