RE: [Exim] Exchange, HELO and underscores

> Block their mail. :) I do so myself, with no handy error message that
> explains the problem... 90% of the mails that comes from ms servers with
> underscores or spaces in the name are spam anyway.

So do I. RFCs are the base of the internet, as they define standards of
interoperability. There would be no internet without interoperability.
Additionally, I usually quote this:

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RFC 1035

<domain> ::= <subdomain> | " "

<subdomain> ::= <label> | <subdomain> "." <label>

<label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]

<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>

<let-dig-hyp> ::= <let-dig> | "-"

<let-dig> ::= <letter> | <digit>

<letter> ::= any one of the 52 alphabetic characters A through Z in
upper case and a through z in lower case

<digit> ::= any one of the ten digits 0 through 9

Note that while upper and lower case letters are allowed in domain names,
no significance is attached to the case. That is, two names with the
same spelling but different case are to be treated as if identical. The
labels must follow the rules for ARPANET host names. They must start with
a letter, end with a letter or digit, and have as interior characters
only letters, digits, and hyphen. There are also some restrictions on
the length. Labels must be 63 characters or less.

RFC 1123
5.2.5 HELO Command: RFC-821 Section 3.5
The sender-SMTP MUST ensure that the parameter in a HELO command is
a valid principal host domain name for the client host. As a result,
the receiver-SMTP will not have to perform MX resolution on this name
in order to validate the HELO parameter.

RFC 2821
4.1.2 Command Argument Syntax
To promote interoperability and consistent with long-standing guidance
about conservative use of the DNS in naming and applications (e.g.,
see section 2.3.1 of the base DNS document, RFC1035 [22]), characters
outside the set of alphas, digits, and hyphen MUST NOT appear in domain
name labels for SMTP clients or servers. In particular, the underscore
character is not permitted. SMTP servers that receive a command in which
invalid character codes have been employed, and for which there are no
other reasons for rejection, MUST reject that command with a 501 response.
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Usually, that suffices to tell people they are doing something wrong
and whatever work it takes to fix it, it's their own fault.

Michael