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My postfix can be set as SMTP server on any email client, on port 25, no auth, to successfully deliver e-mail to any domain listed in $relay_domains. This is bad, and can be easily used to send spam bypassing any restriction set to the submission port (587).

I am surely missing something, hope you can help me.

Here you can grab my postconf -f output (IP's and real hostnames have been replaced):

https://app.box.com/s/asblh2150kmpzr0ag5pp

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    This is expected behavior. For example: Gmail server must connect to port 25 without auth to deliver email from @gmail.com to your relay_domains. – masegaloeh Oct 16 '14 at 10:08
  • I try to explain myself better with an example. If I set up my Thunderbird to use my email server as SMTP with no auth and port 25, I can send email to a known address belonging to my email server. Is this expected behavior? – Gabriele Oct 16 '14 at 10:11
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    Yes. Basically Gmail (in my example) and Thunderbird (in your example) both act as a mail client to send email TO your relay_domains. – masegaloeh Oct 16 '14 at 10:22
  • I have put our discussion in the answer section below. Please consider to accept it so this question doesn't floating in Unanswered Tab – masegaloeh Oct 29 '14 at 1:53
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My postfix can be set as SMTP server on any email client, on port 25, no auth, to successfully deliver e-mail to any domain listed in $relay_domains. This is bad, and can be easily used to send spam bypassing any restriction set to the submission port (587).

This is expected behavior. For example: Gmail server must connect to port 25 without auth to deliver email from @gmail.com to your relay_domains.

I try to explain myself better with an example. If I set up my Thunderbird to use my email server as SMTP with no auth and port 25, I can send email to a known address belonging to my email server. Is this expected behavior?

Yes. Basically Gmail (in my example) and Thunderbird (in your example) both act as a mail client to send email TO your relay_domains.


The only concern is if you accept email from outside your mynetwork (without auth) and then deliver it to domain outside relay_domains. In that case, you have successfully created one free open relay.

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