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Response codes

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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).
    Message 1 of 4 , Apr 30 11:07 AM
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      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 <user@...>:
      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)

      With kind regards,

      Mark Scholten
    • Wietse Venema
      ... 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
      Message 2 of 4 , Apr 30 11:17 AM
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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 <user@...>:
        > 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.

        Wietse
      • Mark Scholten
        ... Hello Wietse, What is the best option to do in your opinion? Both codes are no problem if I understand you correctly. With kind regards, Mark Scholten
        Message 3 of 4 , Apr 30 11:23 AM
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          > -----Original Message-----
          > From: owner-postfix-users@... [mailto:owner-postfix-
          > users@...] On Behalf Of Wietse Venema
          > Sent: Friday, April 30, 2010 8:17 PM
          > To: Postfix users
          > Subject: Re: Response codes
          >
          > 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
          > <user@...>:
          > > 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.
          >
          > Wietse

          Hello Wietse,

          What is the best option to do in your opinion? Both codes are no problem if
          I understand you correctly.

          With kind regards,

          Mark Scholten
        • Wietse Venema
          ... 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
          Message 4 of 4 , Apr 30 11:40 AM
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            Mark Scholten:
            > > 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
            > > <user@...>:
            > > > 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.

            Wietse
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