Holger,
> since upgrading to amavisd-2.4.2 we got a problem
> with spam mails whose subject is not plain ascii.
>
> We use the string "[Maybe Spam]" to tag spam Mails.
> For spam mails with plain ascii subject everything is fine.
> If we receive a spam mail with an subject which is not plain ascii
> the mail is recognised as spam and the subject tag is added.
> But in the process of reformatting the header in sub hdr
> the space in "[Maybe Spam]" is replaced by "\n\t" resulting
> in a line break in the subject header of the mail.
> Subject: [Maybe
> Spam]=?UTF-8?Q?test_=C3=A4l=C3=B6o=C3=BCpj\}=5D\=C2=AC{=CB=9D=C4=B8=...
Well, rfc2822 requires that lines in the header SHOULD not
be longer than 78 characters (excluding CRLF), and prescribes
a way to transparently fold such header fields by inserting
a newline before an existing whitespace character.
Amavisd-new 2.4.2 does just that.
> The MUA's interpret the "[Maybe\n\tSpam]" as "[MaybeSpam]"
> which breaks the spam filter rules at the client side.
> ...The following headers are from a spam mail I set to myself.
> The space in "[Maybe Spam]" is turned into a multi line header.
This is wrong. The "[Maybe\n\tSpam]", when unfolded, gives
the "[Maybe\tSpam]". It is true that amavisd clobbered
a space by turning it into a tab - which is against the rules,
but was done to make people happy (I may reconsider that).
If your MUA interprets it as "[MaybeSpam]", i.e. whitespace lost,
this is clearly wrong way to unfold a header field.
Since Subject is an unstructured header field, it may be
folded at any whitespace, according to rfc2822.
The easiest way to work around the problem is by avoiding
the use of space or tab in inserted text. There is no
provision of 'non-breakable space' character - it is perhaps
something to consider for the future, but I'm not sure how.
Mark
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