>
>I've been running postfix at home and for little projects for a while,
>and decided last week to move to postfix at work too.
>
>I've got a problem there that I hadn't experienced at home.
>
>We seem to get a lot of spam addressed to various addresses that don't
>exist. postfix tries to send a response back saying that the address
>doesn't exist - but it can't, because the "From:" is some host/domain
>that we can't connect to.

Not the "From:" address, the envelope sender address. There is an
important distinction to be made here. Check that you have included
"reject_unknown_sender_domain" in your smtpd_sender_restrictions - this
will stop your system accepting the mail in the first place: it will
reject any mail that doesn't have a valid return address (but it only
checks the domain part).

You may be worried that this will bounce legitimate mail, but if it does,
it's better that the problem is pushed back towards the source - you're
doing them a favour in the long run. If you're worried, have a nightly
cron job that grep's the log and mails you details of any rejected mails
- this way you can contact the admins at any important sites and explain
their problem to them.

There are other steps you can take, but you can leave these until you are
more familiar with postfix.

-- 
Andrew McNamara, Senior Developer, Object Craft
http://www.object-craft.com.au/
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