dig yourdomain.com mx | SMTP commands (HELO, EHLO, MAIL FROM, RCPT TO, DATA, RSET, VRFY, NOOP, QUIT)

Gmail requires TLS to relay mail on your behalf. This type of connection is necessary when your desktop or mobile e-mail client is sending mail from your Gmail account.

However, any normal mail server will still accept a message to a recipient on that server without using TLS or any kind of authentication. For example, you can connect to gmail-smtp-in.l.google.com on port 25 and conduct an unencrypted SMTP session.

Assuming that you are me@mydomain.com, you are logged into your mail server mail.mydomain.com, and you are sending a message to you@yourdomain.com, it looks like this:

1. dig yourdomain.com mx and note the results (let's say it's mail001.yourdomain.com)
2. telnet mail001.yourdomain.com 25
3. Deliver a message:

HELO mail.mydomain.com
MAIL FROM: me@mydomain.com
RCPT TO: you@yourdomain.com
RCPT TO: yourmom@yourdomain.com
DATA

From: "Mehrdad" <me@mydomain.com>
To: "Enrique Peñalosa" <you@yourdomain.com>
Cc: "Gabriela Peñalosa" <yourmom@yourcomain.com>
Date: Fri, 30 Sep 2011 11:21:19 -0700
Subject: Thanks for all the fish!

It was a wonderful picnic. I really enjoyed the salmon burgers.

See you next week,

Mehrdad

.

You issued the HELO command to your SMTP server. This tells it that you want to use the original SMTP commands. STARTTLS is not one of these commands, thus you should not be able to use STARTTLS.

The first step in resolving this is to switch from the HELO command to the EHLO command. When you do, you will see that the SMTP server responds with the set of SMTP extensions that it supports. Hopefully, STARTTLS will be listed.

Here's a conversation with GMail's SMTP server:

CLIENT: EHLO me.example.com
SERVER: 250-mx.google.com at your service
SERVER: 250-SIZE 35882577
SERVER: 250-8BITMIME
SERVER: 250-STARTTLS
SERVER: 250 ENHANCEDSTATUSCODES
CLIENT: STARTTLS
SERVER: 220 2.0.0 Ready to start TLS
<negotiation begins here...>

Basic SMTP Commands

Below are the basic SMTP commands described. All SMTP servers that follows the SMTP protocol specification must support these basic commands. HELO (Hello)The client sends this command to the SMTP server to identify itself and initiate the SMTP conversation. The domain name or IP address of the SMTP client is usually sent as an argument together with the command (e.g. “HELO client.example.com”). If a domain name is used as an argument with the HELO command, it must be a fully qualified domain name (also called FQDN). MAIL FROMSpecifies the e-mail address of the sender. This command also tells the SMTP server that a new mail transaction is starting and makes the server to reset all its state tables and buffers etc. This command is usually sent as the first command after the identifying and login process. If the senders e-mail address is accepted the server will reply with a 250 OK reply code. Example: 

C: MAIL FROM:<mail@samlogic.com> S: 250 OK

 RCPT TO (Recipient To)Specifies the e-mail address of the recipient. This command can be repeated multiple times for a given e-mail message in order to deliver a single e-mail message to multiple recipients. The example below shows how this command can be used to send same e-mail message to two recipients: 

C: MAIL FROM:<mail@samlogic.com> S: 250 OK C: RCPT TO:<john@mail.com> S: 250 OK C: RCPT TO:<peggy@mail.com> S: 250 OK

 
DATAThe DATA command starts the transfer of the message contents (body text, attachments etc). After that the DATA command has been sent to the server from the client, the server will respond with a 354 reply code. After that, the message contents can be transferred to the server. When all message contents have been sent, a single dot (“.”) must be sent in a line by itself. If the message is accepted for delivery, the SMTP server will response with a 250 reply code. Example (the message contents is set to italic in the example below): 

C: DATA S: 354 Send message content; end with <CRLF>.<CRLF> C: Date: Thu, 21 May 2008 05:33:29 -0700 C: From: SamLogic <mail@samlogic.com> C: Subject: The Next Meeting C: To: john@mail.com C: C: Hi John, C: The next meeting will be on Friday. C: /Anna. C: . S: 250 OK

 RSET (Reset)If the RSET command is sent to the e-mail server the current mail transaction will be aborted. The connection will not be closed (this is reserved for the QUIT command, see below) but all information about the sender, recipients and e-mail data will be removed and buffers and state tables will be cleared. VRFY (Verify)This command asks the server to confirm that a specified user name or mailbox is valid (exists). If the user name is asked, the full name of the user and the fully specified mailbox are returned. In some e-mail servers the VRFY command is ignored because it can be a security hole. The command can be used to probe for login names on servers. Servers that ignore the VRFY command will usually send some kind of reply, but they will not send the information that the client asked for. NOOP (No operation)The NOOP command does nothing else than makes the receiver to send an OK reply. The main purpose is to check that the server is still connected and is able to communicate with the client.  QUITAsks the server to close the connection. If the connection can be closed the servers replies with a 221 numerical code and then is the session closed.

 Example - How To Use Basic SMTP Commands The example below shows how some of the basic SMTP commands described in this page can be used to send an e-mail message trough an SMTP server to a recipient. 

S: 220 smtp.server.com Simple Mail Transfer Service Ready C: HELO client.example.com S: 250 Hello client.example.com C: MAIL FROM:<mail@samlogic.com> S: 250 OK C: RCPT TO:<john@mail.com> S: 250 OK C: DATA S: 354 Send message content; end with <CRLF>.<CRLF> C: <The message data (body text, subject, e-mail header, attachments etc) is sent> C: . S: 250 OK, message accepted for delivery: queued as 12345 C: QUIT S: 221 Bye

 In the example above an e-mail message is sent from mail@samlogic.com to john@mail.com. The senders e-mail address is specified by the MAIL FROM command and the recipients e-mail address is specified by the RCPT TO command. The DATA command informs the server that now will the message data be sent (e-mail header, body text etc). The single dot below the message contents informs the SMTP server when the message data ends. After a single dot has been sent to the server and the server has responded, a QUIT command is sent to terminate the session.

 

Part 2:

 Extended SMTP (ESMTP) Commands
 If a client initiates the SMTP communication using an EHLO (Extended Hello) command instead of the HELO command some additional SMTP commands are often available. They are often referred to as Extended SMTP (ESMTP) commands or SMTP service extensions. Every server can have its own set of extended SMTP commands. After the client has sent the EHLO command to the server, the server often sends a list of available ESMTP commands back to the client. EHLO (Extended Hello)Same as HELO but tells the server that the client may want to use the Extended SMTP (ESMTP) protocol instead. EHLO can be used although you will not use any ESMTP command. And servers that do not offer any additional ESMTP commands will normally at least recognize the EHLO command and reply in a proper way. AUTH (Authentication)The AUTH command is used to authenticate the client to the server. The AUTH command sends the clients username and password to the e-mail server. AUTH can be combined with some other keywords as PLAIN, LOGIN and CRAM-MD5 (e.g. AUTH LOGIN) to use different login methods and different levels of security.  The example below shows how AUTH LOGIN can be used to make an authenticated login: 

S: 220 smtp.server.com Simple Mail Transfer Service Ready C: EHLO client.example.com S: 250-smtp.server.com Hello client.example.com S: 250-SIZE 1000000 S: 250 AUTH LOGIN PLAIN CRAM-MD5 C: AUTH LOGIN S: 334 VXNlcm5hbWU6 C: adlxdkej S: 334 UGFzc3dvcmQ6 C: lkujsefxlj S: 235 2.7.0 Authentication successful

 After that the AUTH LOGIN command has been sent to the server, the server asks for username and password by sending BASE64 encoded text (questions) to the client. “VXNlcm5hbWU6” is the BASE64 encoded text for the word "Username" and “UGFzc3dvcmQ6” is the BASE64 encoded text for the word "Password" in the example above. The client sends username and password also using BASE64 encoding. "adlxdkej", in the example above, is a BASE64 encoded username and "lkujsefxlj" is a BASE64 encoded password. More detailed information about the AUTH command is available on this reference page: The AUTH Command.  STARTTLS (Start Transport Layer Security)E-mail servers and clients that uses the SMTP protocol normally communicate using plain text over the Internet. The communication often goes through one or more routers that is not controlled or trusted by the server and client. This communication can be monitored and it is also possible to alter the messages that are sent via the routers. To improve security, an encrypted TLS (Transport Layer Security) connection can be used when communicating between the e-mail server and the client. TLS is most useful when a login username and password (sent by the AUTH command) needs to be encrypted. TLS can be used to encrypt the whole e-mail message, but the command does not guarantee that the whole message will stay encrypted the whole way to the receiver; some e-mail servers can decide to send the e-mail message with no encryption. But at least the username and password used with the AUTH command will stay encrypted. Using the STARTTLS command together with the AUTH command is a very secure way to authenticate users. The example below shows how to combine the STARTTLS and AUTH LOGIN command to make a secure login to an e-mail server (S = Server, C = Client): 

S: 220 smtp.server.com Simple Mail Transfer Service Ready C: EHLO client.example.com S: 250-smtp.server.com Hello client.example.com S: 250-SIZE 1000000 S: 250-AUTH LOGIN PLAIN CRAM-MD5 S: 250-STARTTLS S: 250 HELP C: STARTTLS S: 220 TLS go aheadC: EHLO client.example.com * S: 250-smtp.server.com Hello client.example.com S: 250-SIZE 1000000 S: 250-AUTH LOGIN PLAIN CRAM-MD5 S: 250 HELP C: AUTH LOGIN S: 334 VXNlcm5hbWU6 C: adlxdkej S: 334 UGFzc3dvcmQ6 C: lkujsefxlj S: 235 2.7.0 Authentication successful C: MAIL FROM:<mail@samlogic.com> S: 250 OK C: RCPT TO:<john@mail.com> S: 250 OK C: DATA S: 354 Send message, end with a "." on a line by itself C: <The message data (body text, subject, e-mail header, attachments etc) is sent> S . S: 250 OK, message accepted for delivery: queued as 12345 C: QUIT S: 221 Bye

  *) The client sends the EHLO command again to the e-mail server and starts the communication from the beginning, but this time the communication will be encrypted until the QUIT command is sent. SIZEThe SIZE command has two purposes. The SMTP server can inform the client what is the maximum message size and the client can inform the SMTP server the (estimated) size of the e-mail message that will be sent. The client should not send an e-mail message that is larger than the size reported by the server, but normally it is no problem if the message is somewhat larger than the size informed by the client to the server. The example below shows how a server (S) and client (C) reports size to each other: 

S: 250 SIZE 1000000 C: MAIL FROM:<mail@samlogic.com> SIZE=500000

 The client sends the SIZE command, and size information, together with the MAIL FROM command. The server sends the command and size information alone. The size is always specified in bytes. HELPThis command causes the server to send helpful information to the client, for example a list of commands that are supported by the SMTP server.