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I write exim filters to forward e-mail (like maillist). In filter condition I use header variables. And I faced with some difficultes with $h_subject encoding. Try to explain with example.

New e-mail sent from gmail with subject of few russian words. Of course, subject header is MIME-encoded. It looks like this:

Subject: =?utf-8?Q?[nr=5Fbill]=20=D0=9D=D0=BE=D0=B2=D1=8B=D0=B9=20?=

In Exim filter I want to use next condition: $h_subject: contains "[nr_bill] Новый заказ оформлен"

The problem is that Exim decodes only the first line of header. Decoded header looks like this:

[nr_bill] \320\235\320\276\320\262\321\213\320\271 \n =?utf-8?Q?=D0=B7=D0=B0=D0=BA=D0=B0=D0=B7=20=D0=BE=D1=84=D0=BE=D1=80=D0=BC=D0=BB=D0=B5=D0=BD?=

The first line is successfully decoded and can be used in filters. But the second line is not decoded. And the worst: Exim saves spaces and newline carater beetween lines. As I know, new lines and leadind spaces in headers must be omitted in parsing time.

So $h_subject: contains "[nr_bill] Новый is working well, while $h_subject: contains "[nr_bill] Новый заказ оформлен" not.

Manual says that newline+spaces is normal practice. But it says nothins about multipart MIME-decoding.

Can anyone propound how to resolve this problem or just explain what is the point of those behavior?

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up vote 1 down vote accepted

The answer was found. Thanks to Phil :)

Exim is being very carefully adherent to the MIME specification in RFC2047 which places a maximum length on each encoded-word.

----------------------------8< cut here >8------------------------------

encoded-word = "=?" charset "?" encoding "?" encoded-text "?=" [...]

An 'encoded-word' may not be more than 75 characters long, including 'charset', 'encoding', 'encoded-text', and delimiters. If it is desirable to encode more text than will fit in an 'encoded-word' of 75 characters, multiple 'encoded-word's (separated by CRLF SPACE) may be used.

----------------------------8< cut here >8------------------------------

So the MIME decoder is skipping those long strings and adding them literally.


If you set "check_rfc2047_length" false in Exim's main configuration, the header will be decoded correctly; I would assume (but haven't checked) that Exim would then re-encode things correctly as needed.

check_rfc2047_length = false

So after setting check_rfc2047_length = false all variations of problematic subjects are decoded correctly.

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Although encoded words are limited to 75 characters, you are allowed to put multiple encoded words back to back, if you need to. – james.garriss Mar 4 '14 at 20:01
This worked for me to get proper Exim's Sieve filtering matching against encoded Subject. – Daniel Sokolowski Feb 20 at 23:12

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