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I need to send out notifications from my web-app and I've setup postfix as a null client on my server. So in my /etc/postfix/ I have commented out the lines

#smtp      inet  n       -       -       -       -       smtpd
#local     unix  -       n       n       -       -       local

So after I did that when I try to use telnet, it blocks me out.

$ telnet localhost 25
telnet: Unable to connect to remote host: Connection refused

And when I use sendmail I am able to send emails, which is what I want. So far so good.

Now, I'm concerned about spammers not using my SMTP server as an open relay. To this end, do I need to turn on TLS and SMTP Auth support in postfix or is just disabling network access to postfix (by commenting out those two lines in sufficient?

In other words, in my current setup - with TLS turned OFF and network access to postfix disabled - is there a possibility of my SMTP server being compromised?

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If you have completely disabled access to SMTP, it cannot be used. TLS vs no TLS doesn't make any difference. – grawity Jul 1 '10 at 15:15
The line I commented out is smtp via network interfaces smtp inet . There is another line in the config file which goes like smtp unix which I'm guessing is the one that needs to be enabled to make sendmail work. – ErJab Jul 1 '10 at 17:04
up vote 1 down vote accepted

TLS is used for secure communications with your server. Most spammers and legitimate mailservers would rarely use TLS to send mail to/through your server. It is more common that you would use TLS when sending mail from your email program, i.e. Outlook, Thunderbird, etc.

With network access disabled, you wouldn't be able to send mail from an email client, but, spammers and legitimate mail wouldn't be able to be delivered to your server.

Disabling smtp should work in your situation.

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"legitimate mail wouldn't be able to be delivered to your server." Yeah, I'm using this postfix server to just SEND email. – ErJab Jul 1 '10 at 4:54

I would suggest setting up Postfix with TLS & SMTP Authentication.

This link should explain most of the process on properly securing your mailserver.

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