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Introduction to the SMTP Protocol

Important:  Most SMTP servers have SMTP relay disabled. What this means is that applets that use the email component may not be able to send email to third parties.  In general, the applets can send email to a person with an account on the server they were downloaded from, even if SMTP relaying is denied.  So if your email address is you@somedomain.com, and the applet was downloaded from www.somedomain.com, the SMTP server at somedomain.com will generally accept the email for you@somedomain.com, but not relay the email to somebody@someotherdomain.com.  SMTP servers will also generally relay messages from clients in a certain IP range, so the SMTP class is also usable by code that is running on a server on a LAN with an SMTP server on the LAN (I've used it a lot with servlets with SMTP servers on localhost).

Here is how the protocol looks, as shown in RFC 821.  The email component side of the conversation is shown beside "emc:".  the server side of the conversation is shown beside "serv:".

         serv: 220 somewhere.com Simple Mail Transfer Service Ready
         emc: HELO emc
         serv: 250 somewhere.com

         emc: MAIL FROM:<noone@nowhere.com>
         serv: 250 OK

         emc: RCPT TO:<someone@somewhere.com>
         serv: 250 OK

         emc: DATA
         serv: 354 Start mail input; end with <CRLF>.<CRLF>
         emc: Date: March 17, 2000 10:21
         emc: From: The Sender <noone@nowhere.com>
         emc: To: Some Recipient <someone@somewhere.com>
         emc: Subject: This is just a test
         emc:
         emc: This is a test message
         emc: .
         serv: 250 OK

         emc: QUIT
         serv: 221 SMTP server closing transmission channel

There are five basic phases to this sort of simple transfer of mail.  At the beginning, the server identifies itself, then the client identifies itself (they say hi).  The success message is 250 (250 starts the line).  If you refer to the source code for emc, you will see that it checks to make sure the message returned by the server indicates that everything is ok.  The next two phases are where the client indicates who the mail is from and who the mail is for.  The success message is again 250 OK.  This is where the message could be something indicating a failure, for example, 552 Relay denied.  The next phase is where the email is actually sent.  The header information (date, subject, etc.) is sent first, followed by one blank line, then the message, then a dot "." on a line by itself.  Finally, the connection is closed using a QUIT command.

Attachments could also be sent in the message, but MIME attachments add quite a bit of complication to the email component (not addressed here).

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