How to send an email at the command line with SMTP? Email transaction explained

Without TLS

The port 25 is the default port for a public server and should be configured without TLS

This port is normally only for receiving an email but if you make a connection with the terminal from localhost, it should be permitted to relay the email to the final SMTP server.

Example with:

• telnet

set localecho
set logfile c:\TelnetTest.txt
telnet localhost 25

With TLS

With TSL, you use OpenSsl.

The submission port (ie where you can submit an email) is on:

• 25 with plain text connection and StartTLS
• 465 with SSL
• and nowadays on 587 with TLS

Example:

• On port 465, 587

openssl s_client \
   -connect server.example.com:587 \
   -servername server.example.com \ # the sni
   -crlf
# example with google
openssl s_client -connect smtp.gmail.com:465 -crlf

• On port 25 with STARTTLS

openssl s_client \
   -connect server.example.com:25 \
   -starttls smtp \ # start tls
   -name hostname \ # the hostname given in the EHLO
   -servername server.example.com \ # the sni
   -crlf

where:

• -starttls smtp will start starttls connection for smtp. ie it:
  • will open the connection,
  • receive the banner from the server
  • sends the EHLO command with the hostname with the value of the name option,
  • sends the STARTTLS SMTP command
  • starts the handshake.
• -crlf will convert the LF end of line character from the terminal into the CRLF end of line (this is the end command separator for SMTP)
• -servername is the server name indicator (sni) mandatory in case of SMTP server hosting

Server: Banner

After a successful connection, the SMTP server responds with a banner

220 smtp.eraldy.com ESMTP Postfix

Client: EHLO

The remote SMTP client may introduce itself by giving its fully qualified domain name with the EHLO command (Extended HELO)

EHLO client.example.com

Server: Configuration

The SMTP server answers by advertising its configuration.

250-server.example.com
250-8BITMIME
250-PIPELINING
250-SIZE 10240000
250-AUTH DIGEST-MD5 PLAIN CRAM-MD5
250-CHUNKING
250-ENHANCEDSTATUSCODES
250 SMTPUTF8
...

where:

• 250 means all good
• 8BITMIME means that you can send the data in 8BIT.
• PIPELINING means that you can group the MAIL, RCPT commands together
• SIZE indicates the maximum size of a message.
• AUTH indicates the type of authentication supported (ie the type of SASL mechanisms)
• CHUNKING means that you can send the message not line by line but in chunks of bytes with the BDAT command
• ENHANCEDSTATUSCODES means that the reply will contain an extra status code in the text (known as rfc2034 - Returning Enhanced Error Codes)
• STARTTLS means that you can make use STARTTLS to go from a plain connection to a secure SSL/TLS connection.
• 'SMTPUTF8 means that the server is international and supports UTF8 as character encoding for email address, headers and status.

There are other extensions but they are the common ones.

Pass the message

You give the data with the DATA command. After a successful response from the server, you can copy and paste the message.

Steps:

• Enter the data command

DATA

• The reply should look like that

354 Start mail input; end with <CRLF>.<CRLF>

• Copy, paste the message and enter. Note that the message is overly simplified. In the real world, with an SMTP client, it would have more headers (such as content type, date, …). The server should apply the defaults (ie ASCII encoding, send date as now, …)

Subject: Hey!

We should meet up! ;)

• Terminate with a single point and enter

.

• If the email was accepted, a 250 reply should be received.

250 Ok