Some of the key components defined in RFC 5321 include:
Mail Commands: These are the commands used to initiate the email sending process, such as HELO, MAIL FROM, and RCPT TO. Reply Codes: These are the numerical codes returned by the server to indicate the status of the email transaction. Error Handling: Guidelines on how to handle transient and permanent errors during the email sending process. Data Transmission: Rules for the transmission of email message data between servers. Extensions: Mechanisms for extending SMTP functionality through the use of Extended SMTP (ESMTP).