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> > Mark Scholten:
> > > Hello,
> > >
> > > Should a system that gives the following response to a SMTP request
> > give a
> > > code 450 error or a code 550 error (I did think a code 550 should be
> > given).
> > >
> > > (host mxfilter4.domain.net[70.34.y.z] said: 450 4.1.1
> > <email@example.com>:
> > > Recipient address rejected: undeliverable address: host
> > > mail.domein.nl[70.34.x.y] said: 550 "Unknown User" (in reply to RCPT
> > TO
> > > command) (in reply to RCPT TO command))
> > >
> > > I did change domain names and the email address and IP addresses (as
> > they
> > > aren't needed in this example and pointing to someone is also
> > something I
> > > don't want to do).
> > >
> > > Is what I suspect correct or are I wrong?
> > > Current situation:
> > > - it gives a 450 error code (while it receives a 550 error code)
> > > I think it should be:
> > > - it should give a 550 error code (because it did receive a 550 error
> > code
> > > from a server downstream)
> > You can configure Postfix to reject such mail or to reply with
> > 4xx. In the latter case you get a chance to make a whitelist entry
> > for sites that send legitimate mail with a non-replyable address.
> Hello Wietse,
> What is the best option to do in your opinion? Both codes are no problem if
> I understand you correctly.
In your case you are sending a recipient address verification probe
to a down-stream system. Here, replying with 5xx is appropriate,
once you have verified that the setup is working properly.
In the case of up-stream (sender) address verification, not doing
it at all is the appropriate choice for most sites. Up-stream
verification is practical only for a site with very little traffic.