Mail expn vrfy etrn during connection to mta

A blast from the past!

The commands you mentioned are related to the Simple Mail Transfer Protocol (SMTP) and are used to interact with a Mail Transfer Agent (MTA) during a connection.

Here's a brief explanation of each command:

  1. MAIL EXPN:
    • MAIL is a command that initiates a mail transaction.
    • EXPN is a subcommand that expands a mailing list. It takes a list name as an argument and returns a list of email addresses that are members of that list.
  2. VRFY:
    • VRFY is a command that verifies the existence of a user on a remote system. It takes an email address as an argument and returns a response indicating whether the user exists or not.
  3. ETRN:
    • ETRN is a command that enables or disables the "Enhanced Delivery" feature. This feature allows the MTA to buffer messages and deliver them later, rather than immediately.
  4. ETRN during connection to MTA:
    • When you connect to an MTA using SMTP, you can issue the ETRN command to enable or disable Enhanced Delivery. This can be useful in certain situations, such as when you need to buffer messages before delivering them.

Here's an example of how you might use these commands in an SMTP session:

$ telnet mail.example.com 25
Trying 192.0.2.1...
Connected to mail.example.com.
Escape character is '^]'.
220 mail.example.com ESMTP
HELO client.example.com
250 mail.example.com
MAIL FROM:<[email protected]>
250 2.1.0 Ok
RCPT TO:<[email protected]>
250 2.1.5 Ok
EXPN mailing-list
250 2.1.5 Ok
<[email protected]>, <[email protected]>,...
VRFY [email protected]
250 2.1.5 Ok
ETRN
250 2.0.0 Ok

In this example, we connect to the MTA using telnet, issue the HELO command to identify ourselves, and then use the MAIL FROM and RCPT TO commands to initiate a mail transaction. We then use the EXPN command to expand a mailing list, the VRFY command to verify the existence of a user, and finally the ETRN command to enable Enhanced Delivery.