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I wanted to host my own mail server and in order to do so I've installed Postfix 2.7.0 and Dovecot 1.2.9 on Ubuntu Server 10.04. I had fun reading the docs and configuring all these great tools, I am also hosting locally a DNS server and a Web server but right now I am stuck on something regarding Postfix.

One big problem I face right now is that Google SMTP servers reject my mail ONLY WHEN it is being sent remotely. Strange, right?

So if I telnet localhost 25, issue AUTH LOGIN(not an open relay of course) then send the mail I get a status = sent and I actually receive the mail message in my Gmail's account Inbox. Works like a charm.

On the other hand, if I want to send the mail from a remote host, I use

openssl s_client -connect mail.mydomain.com:465

Then I issue AUTH LOGIN, the login process is successful. So I don't have a problem communicating with my server. I can see what happens in the log files. But when I try to send mail like this, Google replies with:

Our system has detected an unusual rate of 421-4.7.0 unsolicited mail originating from your IP address. To protect our 421-4.7.0 users from spam, mail sent from your IP address has been temporarily 421-4.7.0 rate limited.

Why Google accepts my mail if being sent locally but rejects it when I openssl to my server and send it remotely? It's like Postfix on my server makes Google aware of how the email it forwards got to it.

Any ideas?

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The IP of the host mail.mydomain.com is sending too many mails, unsolicited emails which is worse, too fast. This is clear from the error message:

Our system has detected an unusual rate of 421-4.7.0 unsolicited mail originating from your IP address. To protect our 421-4.7.0 users from spam, mail sent from your IP address has been temporarily 421-4.7.0 rate limited.

So, investigate if there has been a security breach in your host, or if there are other host that shares your IP which have problems.

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  • I don't recognise the 421-4.7.0 syntax. Commented Aug 9, 2014 at 11:56
  • @FaheemMitha it's possibly an alias for something else
    – Braiam
    Commented Aug 9, 2014 at 12:13
  • OK, @Braiam but why it DOES work when I send the mail from localhost, using telnet localhost 25? I mean, the IP of the mail server is the same! And Google does not reject it this way.
    – Terente
    Commented Aug 9, 2014 at 12:58
  • The "does work" vs "doesn't work" is probably a red herring. The 421 syntax comes from line breaks in the SMTP response message protocol.
    – Jon Watte
    Commented Aug 5, 2016 at 16:15

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