SMTP PIPELINING Correctness when using unrecognized command - smtp

Question

While understanding SMTP PIPELINING RFC 2920 Section 3.2 indicates that a server supporting pipelining MUST NOT buffer responses to unrecognized commands. Is the ideal response to expect an SMTP Protocol Violation Error?
For example, here's a transcript without pipelining but when using an UNKNOWNCOMMAND:
C: EHLO domain
S: 250-ServerName ..... 250-PIPELINING
C: UNKNOWNCOMMAND
S: 500 Unrecognized Command
C: EHLO domain
S: 250-ServerName ..... 250-PIPELINING
With pipelining
C: EHLO domain
S: 250-ServerName ..... 250-PIPELINING
C: UNKNOWNCOMMAND\r\nEHLO domain
S: 500 Unrecognized Command
S: 250-ServerName ..... 250-PIPELINING
Is this the expected behavior or should the SMTP server not buffer responses to unknown commands and to specific commands like EHLO | DATA | NOOP (RFC 2920 Section 3.2 Point 5)? So for the pipelined UNKNOWNCOMMAND\r\nEHLO domain, should the server send only one 500 Unrecognized Command response and flush the TCP input buffer? Are the servers which exhibit this behavior adhering to a stricter constraints?

Answer 1

The relevant points of section 3.2 are:
(6) [The server] MUST NOT buffer responses to unrecognized commands.
(9) [The server] MUST NOT flush or otherwise lose the contents of the TCP input buffer under any circumstances whatsoever.
I interpret these two points as follows: If you send an unknown command followed by a known command in a "single" TCP packet, the server has to send its response to the unknown command immediately before it reads from its input again. In other words, you receive "two" TCP packets instead of one. Unless you monitor your network traffic (or measure the precise timing programmatically), you won't be able to notice the difference. As indicated by the point 9 above, the server may not reset its input buffer.
As a client, you can usually also send commands after EHLO and DATA immediately without any issues because most server don't look ahead in their input buffers. However, such a client violates the pipelining extension. Detecting this can be used as an anti-spam technique (see invalid pipelining in this Wikipedia article).