From: mouss (moussnetoyen.net)
Date: Thu Apr 17 2008 - 06:48:59 CDT

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Florian Effenberger wrote:
> Hello,
>
> I currently work on my Postfix configuration and I came across a
> phenomenon that I don't know how to deal with. I have set
>
> local_recipient_maps = $alias_maps unix:passwd.byname
>
> so unknown recipients get rejected directly in the SMTP session.
> However, some of my users forward their e-mail to another machine.
> Let's take johnmysite.com (on my MX) forwards his mail to
> johnhissite.com (on another machine I don't control).
>
> When paulthirdsite.com mails to johnmysite.com, and johnhissite.com
> bounces (like for spam protection or other errors), my MX sends a
> bounce message to paulthirdsite.com. Theoretically, this could be
> used for backscattering, I guess?

yes. if spammer forges addresses and sends mail to john, that's what
will happen.
>
> Is there anything I can do about it? Is forwarding e-mail generally a
> bad habit?

it's becoming harder than it was...

- you should never forward mail that may be caught by a "reasonable"
spam filter. for example, if you use SA (with network test and Bayes),
then forwarding a message that has a score >= 4 is probably not
recommended.

- you should watch logs and disable the forwarding if you notice a "storm".

- maybe throttling bounces would be a good idea.

PS. you should never bounce to header addresses. (postfix behaviour is
correct).

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