Extracted from: https://serversmtp.com/smtp-commands/
HELO It’s the first SMTP command: is starts the conversation identifying the sender server and is generally followed by its domain name.
EHLO An alternative command to start the conversation, underlying that the server is using the Extended SMTP protocol.
MAIL FROM With this SMTP command the operations begin: the sender states the source email address in the “From” field and actually starts the email transfer.
RCPT TO It identifies the recipient of the email; if there are more than one, the command is simply repeated address by address.
SIZE This SMTP command informs the remote server about the estimated size (in terms of bytes) of the attached email. It can also be used to report the maximum size of a message to be accepted by the server.
DATA With the DATA command the email content begins to be transferred; it’s generally followed by a 354 reply code given by the server, giving the permission to start the actual transmission.
VRFY The server is asked to verify whether a particular email address or username actually exists.
TURN This command is used to invert roles between the client and the server, without the need to run a new connaction.
AUTH With the AUTH command, the client authenticates itself to the server, giving its username and password. It’s another layer of security to guarantee a proper transmission.
RSET It communicates the server that the ongoing email transmission is going to be terminated, though the SMTP conversation won’t be closed (like in the case of QUIT).
EXPN This SMTP command asks for a confirmation about the identification of a mailing list.
HELP It’s a client’s request for some information that can be useful for the a successful transfer of the email.
QUIT It terminates the SMTP conversation.