Proper response to SMTP HELO - email

Question

The following is the beginning of an SMTP transaction example shown in the
textbook Computer Networking (6th international edition):
S: 220 hamburger.edu
C: HELO crepes.fr
S: 250 Hello crepes.fr, pleased to meet you
The S: prefix indicates that it is a line sent by the server, C: that it is
a line sent by the client. The Wikipedia page SMTP has an SMTP
example that has a similar response to HELO.
Is the server's response to HELO spec compliant? RFC 5321
specifies the server's response to HELO/EHLO thus:
ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF )
/ ( "250-" Domain [ SP ehlo-greet ] CRLF
*( "250-" ehlo-line CRLF )
"250" SP ehlo-line CRLF )
As I understand the spec, the server's response in the above example should be
250 hamburger.edu
That is, it should respond with 250 followed by its own hostname, not the
client's hostname, and certainly not the arbitrary greeting message shown
in the example.
What is the proper response to HELO?
Is the Computer Networking example incorrect?

Answer 1

The short answer
The example is not valid due to the stray Hello just after the 250. But.. that probably doesn't matter.
The long(er) answer
First let's look at the syntax for the client's "HELO":
"HELO" SP Domain CRLF
The client sends HELO followed by a space, followed by his domain name, followed by a CRLF. How do I know that it's the clients domain? Well:
The argument clause contains the fully-qualified domain name of the
SMTP client, if one is available
Now the response:
ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF )
/ ( "250-" Domain [ SP ehlo-greet ] CRLF
*( "250-" ehlo-line CRLF )
"250" SP ehlo-line CRLF )
There are two options here:
"250" SP Domain [ SP ehlo-greet ] CRLF
( "250-" Domain [ SP ehlo-greet ] ...
Both of them don't fit since the domain name is expected instead of the Hello we see.
The following ehelo-greet part is fine though. It is explained on the next page of the RFC:
ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127)
; string of any characters other than CR or LF
So it can be any string that doesn't contain \r or \n. The string , pleased to meet you clearly falls into that category.
Why it doesn't matter
Having said all of that, note that the fact that the example is invalid, doesn't mean that it doesn't happen in practice or that a server that would send such an example wouldn't work properly. There's a difference between theory and practice. For instance, if we look at Java's official JavaMail, in SMTPTransport.java, on lines 1662-1665 we find the following code:
if (first) { // skip first line which is the greeting
first = false;
continue;
}
This is from a method named ehlo(String domain) which handles sending and receiving the HELO command. JavaMail skips the entire greeting line, and I suspect that other client could be doing the same.

Answer 2

RFC 821 didn't actually specify what the response to HELO is, but all the examples used the recipient's domain as the body of the message. Later RFCs have defined extended hello (EHLO) which actually has a defined format to the response. But given the lack of clarity in the original standard, practically speaking anything between the 250 and the CR is kosher when the client is so careless as to send HELO instead of EHLO.