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Why is the requirement not to append extra info at the end of EHLO is only a SHOULD NOT? #107

Open aamelnikov opened 2 hours ago

aamelnikov commented 2 hours ago

Paragraph 2 of Section 4.1.1.1 says:

RFC 2821, and some earlier informal practices, encouraged following the literal by information that would help to identify the client system. That convention was not widely supported, and many SMTP servers considered it an error. In the interest of interoperability, it is probably wise for servers to be prepared for this string to occur, but SMTP clients SHOULD NOT send it.

Ted Lemon wrote:

Non-DNS question: why is the "SHOULD NOT" in paragraph 2 of 4.1.1.1 not a MUST NOT? What is the exception?

aamelnikov commented 2 hours ago

John Levine replied:

I don't think I've ever seen an EHLO with extra data. I looked at the code for my mail server (mailfront) and it doesn't anticipate extra data in EHLO at all. So I think we can say MUST NOT now.

As you note, MTAs are always free to make their own decisions about what to accept but this seems very low risk.