The RFC number for the current SMTP standard is RFC 5321.
Published in October 2008, RFC 5321 specifies the basic Simple Mail Transfer Protocol used to transport email between mail servers. It describes the client-server conversation, including commands such as `EHLO`, `MAIL FROM`, `RCPT TO`, and `DATA`, along with response codes and delivery behavior. SMTP’s central job is moving mail toward its destination, often through one or more relays.
RFC 5321 replaced RFC 2821 and obsoleted the much older RFC 821, which documented the original 1982 SMTP specification. RFC 5322 is a neighboring standard, not a replacement: it defines Internet message format, including headers and bodies, while RFC 5321 defines transport.
The distinction matters because SMTP extensions are specified in additional RFCs, and authenticated submission is commonly handled using related submission standards and extensions. A revision known as SMTP 5321bis has been progressing through the IETF, but an Internet-Draft is not itself a published RFC. Thus 5321 remains the canonical RFC answer for this quiz question.