Understand end of transmission of email message in SMTP

Question

I have a log for a simple SMTP transaction. (Its a bit long so access it here ). It shows the exchange of low-level handshakes such as HELO , MAIL , etc.

When the server which is supposed to receive an email is ready to accept email data, it send the following (line 12 in the log):

RawSmtp Information: 0 : << 354 Start mail input; end with <CRLF>.<CRLF>

However, the end of transmission is a bit confusing (lines 74/75):

RawSmtp Information: 0 : >> cannot foresee
RawSmtp Information: 0 : >> 
RawSmtp Information: 0 : << 250 OK

which when viewed in notepad looks like:

The code however is simply written like this:

                            WriteResponse(writer, "354 Start mail input; end with <CRLF>.<CRLF>");
                            string line;
                            while ((line = reader.ReadLine()) != null && line != ".")
                            {
                                //do stuff
                            }

Is that standard? I mean sending something like <CRLF>.<CRLF> and only checking for the dot ( . )...?

Answer 1

It is checking for a whole line with just a . in it. This is the standard technique for indicating the end of a DATA section. See here .

Also, as @jstedfast points out, the StreamReader.ReadLine() method removes the "\\r\\n" end-of-line sequence from the strings that it returns, so in that case, the resulting string will be "."