Valid SMTP Commands According to RFC 821

BC.

SMTP (Simple Mail Transfer Protocol) is a protocol used for email communication. It is defined in RFC 821, which specifies the commands and responses used in the protocol.

A. EHLO and B. HELO are both valid SMTP commands according to RFC 821.

1. HELO: This command is used by the SMTP client to identify itself to the SMTP server. The syntax for this command is: "HELO domain.name", where "domain.name" is the domain name of the SMTP client. For example, "HELO example.com".

2. EHLO: This command is similar to the HELO command, but it includes an extended greeting that lists the capabilities of the SMTP client. The syntax for this command is: "EHLO domain.name", where "domain.name" is the domain name of the SMTP client. For example, "EHLO example.com".

C. RCPT and D. AUTH are not valid SMTP commands according to RFC 821.

1. RCPT: This command is used in the SMTP conversation to specify one recipient for the email message. However, it is not a valid command according to RFC 821, but rather to RFC 2821, which obsoleted RFC 821.

2. AUTH: This command is not a valid command in the SMTP conversation according to RFC 821. AUTH is defined in RFC 4954 as a command that can be used to authenticate SMTP clients to servers, but this RFC updates RFC 2554, which is not related to SMTP protocol.

In summary, the two valid SMTP commands according to RFC 821 are HELO and EHLO.