Re: [exim] Exim and DSN's

On Thu, 20 Feb 2014, Ian Coetzee wrote:
> Upon investigation I see that exim does not support DSN's derectly but
> rather via a patch. However if I read it correctly the patch only extends
> to 4.72.

The only Exim DSN patch I'm aware of is for 4.76.

However, even in 4.76 it is broken. The DSN RFC explicitly requires all
servers offering the DSN option to use MIME bounce messages, and the patch
doesn't add that feature to Exim.

Installing it is harmful, since it prevents a DSN-supporting client from
recognizing Exim as non-standard and taking appropriate action.


DSN isn't really a sensible extension anyway. It wouldn't be too bad if it
was possible to decline DSN when an SPF check is inconclusive (meaning that
the DSN you promise to send might be backscatter), but you can't. You have
to commit to offering it at EHLO and don't have the needed information until
MAIL FROM. It is explicitly forbidden to base your acceptance/rejection
decision on whether the sender tried to use the extension.

So, the only RFC-legal options are:

1. Advertise the DSN ESMTP extension to no one.

2. Refuse all mail that does not have an SPF pass (regardless of whether the
sender actually wanted to use DSN).

3. Run a server that can be forced to emit backscatter.

I think #1 is clearly the only reasonable choice.

---- Michael Deutschmann <michael@???>