Question

I need to implement support of multiple messages per one connection for my SMTP server.

Every message ends with:

data
<<content>>
.

And it’s logically that protocol state should be reset to “after receive authentication” point. Is it correct?

The question: Is it possible that any client sends message content with multiple data commands? Does the standard allow it?

Answer 1

From RFC2821 ("Simple Mail Transfer Protocol"):

The mail data is terminated by a line containing only a period, that
is, the character sequence "." (see section 4.5.2).

…

Receipt of the end of mail data indication requires the server to process the stored mail transaction information. This processing consumes the information in the reverse-path buffer, the forward-path buffer, and the mail data buffer, and on the completion of this command these buffers are cleared.

i.e. after <CRLF>.<CRLF> is received, the server consumes the mail data and clears its buffers; hence the client cannot then send more content associated with the message, since the server will have forgotten about the message.

…

Once started, a mail transaction consists of a transaction beginning command, one or more RCPT commands, and a DATA command, in that order.

…

MAIL (or SEND, SOML, or SAML) MUST NOT be sent if a mail transaction is already open, i.e., it should be sent only if no mail transaction had been started in the session, or it the previous one successfully concluded with a successful DATA command, or if the previous one was aborted with a RSET.

i.e. MAIL begins a new mail transaction, and a successful DATA command (terminated by <CRLF>.<CRLF>) concludes it; the client may then send another message.

---

From RFC4954 ("SMTP Service Extension for Authentication"):

After an AUTH command has been successfully completed, no more AUTH commands may be issued in the same session. After a successful AUTH command completes, a server MUST reject any further AUTH commands with a 503 reply.

i.e. authentication takes place at most once per session, and applies until the end of that session.