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Sat, Apr 04, 2015 11:20 pm

Testing email delivery with SMTP commands

Telnet can be used from a command line interface, aka shell prompt, to manually send commands to an Simple Mail Transfer Protocol (SMTP) server to test email deliveries. An SMTP server typically listens on port 25 for incoming connections from mail clients or other email servers. So you could enter telnet mail.example.com 25 to connect to that port on a server, e.g., mail.example.com in this example, from a command line.

Telnet is available on Linux systems, or can be installed if it is not, and is also available on Mac OS X systems where you can run the application by using the Terminal application to obtain a shell prompt. Telnet was available by default with early versions of Microsoft Windows, but that is no longer the case. The free and open source PuTTY program is an excellent telnet client for Windows, however. To use it to connect to port 25 on an email server, you simply select Telnet as the connection type and put 25 in the port field.

Once you've connected to port 25 on a mail server, the first command to enter is either helo or ehlo. Issuing that command is akin to saying "hello" when you meet someone. The command is usually followed by the name of the system from which you are connecting, e.g., helo me.example.com if you ran the telnet command on a system with a fully qualified domain name (FQDN) of me.example.com. But you can follow the helo or ehlo command with anything, just as you could say "hello, I'm Paul" when you met someone even if your name was Peter.

Ehlo is a command that is part of Extended SMTP (ESMTP), aka "Enhanced SMTP", which defines protocol extensions for the SMTP protocol. Ehlo is shorthand for "Extended Hello" and in the protocol extensions of ESMTP provides a similar role of an introduction. If the email server to which you are connecting supports the protocol extensions of ESMTP, it will reply with a list of the keywords it supports with each preceded by the code 250 as shown below:

$ telnet smtp.mandrillapp.com 25
Trying 54.158.189.65...
Connected to smtp.mandrillapp.com.
Escape character is '^]'.
220 smtp.mandrillapp.com ESMTP
ehlo moonpoint.com
250-ip-10-81-192-73
250-PIPELINING
250-SIZE 26214400
250-STARTTLS
250-AUTH PLAIN LOGIN
250-ENHANCEDSTATUSCODES
250 8BITMIME

If the server doesn't support ESMTP, it will return error code 500, instead. ESMTP client software can then try either HELO, instead, or issue the QUIT command to terminate the connection to the server.

If the email server doesn't require authentication, you can then issue commands specifying the "from" and "to" addresses for the email message as shown below:

$ telnet mail.example.com 25
Trying 192.168.101.222...
Connected to mail.example.com.
Escape character is '^]'.
220 mail.example.com ESMTP Sendmail 8.14.7/8.14.7; Sat, 4 Apr 2015 22:02:39 -0400
mail from: testing@example.com
250 2.1.0 testing@example.com... Sender ok
rcpt to: moonpoint@example.com
250 2.1.5 moonpoint@example.com... Recipient ok
data
354 Enter mail, end with "." on a line by itself
This is a test.
.
250 2.0.0 t3520758032350 Message accepted for delivery
quit
221 2.0.0 mail.example.com closing connection
Connection closed by foreign host.

First you issue the command mail from: followed by the email address that is the "from" address. You then issue the command rcpt to: followed by the destination email address. The content of the message is specified by typing data then hitting Enter and then typing the contents of the email you wish to send. To terminate the content, type a period by itself as the first character on a line and then hit Enter. That's all you need to send an email message and you can then terminate the connetion to the server with the Quit command.

In the example above, the message you send won't have a subject, because none was specified. You can specify the subject by entering Subject: after the Data command. You can also include From: and To: in the data that follows the data command. Those don't even have to be the same as what you speficied with the mail from: and rcpt to commands. If you enter a From: and To: address after the data command, those will be what the recipient sees when he or she views the message. E.g., you could use rcpt to: mary@example.com to send an email to mary@example.com, but put a bogus address after the data command, such as From: bogusemail@nonexistent.com and the email would still be delivered successfully to Mary's mary@example.com email address, but when she viewed the message it would appear to her it was sent to bogusemail@nonexistent.com.

$ telnet mail.example.com 25
Trying 192.168.101.222...
Connected to mail.example.com.
Escape character is '^]'.
220 mail.example.com ESMTP Sendmail 8.14.7/8.14.7; Sat, 4 Apr 2015 22:17:16 -0400
mail from: testing@example.com
250 2.1.0 testing@example.com... Sender ok
rcpt to: someone@example.com
250 2.1.5 someone@example.com... Recipient ok
rcpt to: someoneelse@somesite.com
250 2.1.5 someoneelse@somesite.com... Recipient ok
data
354 Enter mail, end with "." on a line by itself
From: someone@example.com
To: bogusemail@example.com
Subject: A Test
Just a test.
.
250 2.0.0 t352HGPe003192 Message accepted for delivery
QUIT
221 2.0.0 mail.example.com closing connection
Connection closed by foreign host.
$

In the example above, I skipped the introduction using helo or ehlo and went straight to rcpt to. I then entered rcpt to: someone@example.com. When I entered the rcpt to: someoneelse@somesite.com command I wasn't adding a second delivery destination for the email but replacing the prior email recipient; you can just re-enter commands if you've made typos. I then changed the "from" address that the recipient would see with From: someone@example.com and also inserted To: bogusemail@example.com. There's no validation of the from address at any part of this process, which shows just how easy it is to spoof the "from" address in an email message, which is why you should never assume that the "from" address in an email message reflects the actual sender. Spammers and malware distributors often insert fake "from" addresses in their email. The email server will check that the rcpt to: address is a valid email address on the server or the sender is allowed to "relay" email based on the sender's IP address, email address, or authentication, but the "to" address seen in a message when viewed in an email client also doesn't necessarily reflect the one used in the rcpt to address.

If the rcpt to address isn't a valid email account on the server and if the server isn't configured to "relay" email to addresses that exist on other servers, or at least not for the sender of a particular message, it will respond with a "relaying denied" message.

s telnet example.com 25
Trying 192.168.101.222...
Connected to example.com.
Escape character is '^]'.
220 localhost.localdomain ESMTP Sendmail 8.14.7/8.14.7; Sat, 4 Apr 2015 22:15:24 -0400
ehlo example.com
250-example.com Hello me.somewhere.com [10.0.1.12], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE
250-DSN
250-ETRN
250-AUTH DIGEST-MD5 CRAM-MD5
250-DELIVERBY
250 HELP
mail from: testing@example.com
250 2.1.0 testing@example.com... Sender ok
rcpt to: someone@someothersite.com
550 5.7.1 someone@someothersite.com... Relaying denied

The above commands can be used by one email server communicating with another, e.g., a sender's email server communicating with a recipient's email server. They can also be used by a client email application, e.g., Thunderbird or Microsoft Outlook. If you are sending email from an email client to an email server, you may be required to authenticate with that server first with a userid and password before the server will allow the client to transmit email through it,though. This is a mechanism to block spammers from sending email via the server. You can use the AUTH PLAIN command to transmit those credentials if the email server supports that command. E.g., in the example below the server supports that command, since it returns the code 250 followed by "AUTH PLAIN" after an ehlo command is issued, so I can authenticate with it by issuing the command AUTH PLAIN followed by the appropriate credentials.

$ telnet smtp.mandrillapp.com 25
Trying 54.204.65.134...
Connected to smtp.mandrillapp.com.
Escape character is '^]'.
220 smtp.mandrillapp.com ESMTP
ehlo test
250-ip-10-187-29-39
250-PIPELINING
250-SIZE 26214400
250-STARTTLS
250-AUTH PLAIN LOGIN
250-ENHANCEDSTATUSCODES
250 8BITMIME
AUTH PLAIN AG1vb25wb22udEBob3RtYW2sLmNvbQBDaTMtY2RfOT0McklOd23OMFEsNGFB
235 2.7.0 Authentication successful
Mail From: testing@example.com
250 2.1.0 Ok
RCPT To: moonpoint@somesite.com
250 2.1.5 Ok
Data
354 End data with <CR><LF>.<CR><LF>
This is a test message.
.
250 2.0.0 Ok: queued as 85761C042C
quit
221 2.0.0 Bye
Connection closed by foreign host.

So what is that long string of characters after the AUTH PLAIN command. That's actually the userid and password that the server requires for authentication. It looks like just random letters and numbers because the userid and password are base64-encoded. You can generate the encoded credentials using a Perl command similar to the one below:

$ perl -MMIME::Base64 -e 'print encode_base64("\000moonpoint\@example.com\000SomePassword");'
AG1vb25wb2ludEBleGFtcGxlLmNvbQBTb21lUGFzc3dvcmQ=

The command is in the format perl -MMIME::Base64 -e 'print encode_base64("\000user_name\@domain_name\000Password");'. The user name is preceded by \000, i.e., a backslash followed by a null character represented by three zeros. If authentication requires that you enter a userid in the form of userid@domainname, instead of just username, then you need to precede the "@" with the backslash escape character. If you don't do so, Perl will think the "@" represents a reference to an array and the result you get won't work. You then add another backslash followed by three zeros and the password. The encoded data will then be returned. In this case it was AG1vb25wb2ludEBleGFtcGxlLmNvbQBTb21lUGFzc3dvcmQ= which is what I would copy and paste after the AUTH PLAIN command when providing credentials to the server. Note: you can also use online services to do the base64 encoding, but you would be providing the login credentials to get the results, so someone else could possibly learn the userid and password.

References:

  1. SMTP, testing via Telnet
    Last modified: June 11, 2013
    FreeBSDwiki

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