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 > > <u...@domein.nl>: > > > 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