Bug#821830: exim4: Exim rejects mail with overlong lines without any explanation

[Andreas Metzler]

On 2016-04-19 Francesco Potortì <Potorti at isti.cnr.it> wrote:
> Package: exim4
> Version: 4.87-1
> Severity: normal

> I use Exim to get mail from Fetchmail.  In the last days I have lost
> some mails and got unsubscribed from some mailing lists.  After much
> digging, I discovered that an ACL rule has now been added which limits
> line length to 998.

> There are several problems here.

> 1) mail is rejected without any explanation; as a non-expert of Exim, it
>    took me much digging to discover that an ACL rule exists which
>    rejects mail without any explanation added to the logs: as a minimum,
>    a message line should be added to the acl rule

> 2) this change is very important, as it can cause people to lose mail:
>    the administrator should be clearly notified of it upon upgrade

> 3) there should be an easy way to disable the new behaviour, in the form
>    of a configuration macro

> 4) apparently people around there send legitimate email with overlong
>    lines.  Since Exim does not suffer from it, it should apply Postel's
>    law «be conservative in what you do, be liberal in what you accept
>    from others»: mail should definitely not be rejected by default
>    because it contains overlong lines

Hello,

Accepting overling lines is not a solution. - Exim must not *send* *out*
overlong lines, and therefore cannot accept these messages in the first
place. This is a rfc MUST and can cause loss of correct mail.
Quoting <https://bugs.exim.org/show_bug.cgi?id=1684>:
| In addition, MANY MTAs (including gmail) will respond to an over-length
| line by hanging up on the connection (TCP RST) without any error
| message. Exim misclassifies this as a host error (as documented in
| http://www.exim.org/exim-html-current/doc/html/spec_html/ch-smtp_processing.html#SECToutSMTPerr)
|
| As a result, sending messages that contain long header lines to a local
| server for delivery to a remote site can interrupt delivery of legitimate
| messages to that remote site. This has been seen with certain "References"
| headers.

1) is fixed in exim GIT <https://bugs.exim.org/show_bug.cgi?id=1817> and
will be in the next upstream version.

cu Andreas
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