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Re: Problems with mail system? [Fwd: Returned mail: User unknown]

On Thu, Sep 07, 2000 at 06:09:31PM +1100, Craig Sanders wrote:
> nobody's telling anyone to get any particular ISP or that they have to
> pay for a premium quality service.


> it's simple - if you want a service that's worth having, you pay
> whatever it costs. if you don't want that, then pay for a cheap/crappy
> service and put up with it without whining.


> (that said, i don't believe that missing reverse DNS is a good
> reason for bouncing mail. a "450 try again later" response is more
> appropriate, to cope with temporary dns outages. bouncing mail from
> nonexistant domains, however, is a different story - it's almost
> certainly spam and there's no point in accepting a message which
> doesn't have a valid reply address so just bounce it)

Ouch.  I think debian developers should have a better understanding
of DNS.

[1] A mail domain does not have to have a valid IP address.

As a default, if you use a mail domain for which there's no mail exchange,
the default is to look for a host address with that name.  But that's
just the default.

[2] A PTR record does not have to contain *any* information whatsoever.

Imagine the mail client at initiates an smtp session with
your system.  Your mail server performs a PTR lookup and gets back  It then performs an A lookup and finds that has the address  What have you learned?


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