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I found that my server's sent mail can not reach to the mail receipent in yahoo gmail etc..

After studies, I have found that I need to check

  1. port 25

  2. Reverse DNS, that means the receiver's MTA can use my domin name to check whether my server actually exist?

So the problems are:

  1. Are there anythings i have to do before i can send the mail in those popular mail server? I need to register a mx record?

  2. Would somebody kindly breifly explain about reverse dns?

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2  
For the port 25 check, you need to make sure your ISP permits traffic from your server to the rest of the Internet. To do this, try telnet gmail-smtp-in.l.google.com 25. This will create a TCP connection to GMail's MTA on the SMTP port. If you get a response (and you can type "EHLO example.com" and get something back), your ISP is permitting that traffic, and your problem is elsewhere. If the telnet hangs, then you need to check with your ISP. –  cjc Mar 21 '12 at 16:35
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In addition to cjc's comment, if youre sending from a home server, youre also likely on the spamhaus block list (spamhaus.org/pbl) which contains most dynamic IPs in the world. The only way around this is to relay through another mail server you are permitted to use. –  Patrick Mar 21 '12 at 17:50

2 Answers 2

It's hard to tell where your mail is failing, what services/etc are you using? Any error messages?

A few things to consider:

1) Make sure you have an MX record pointing to a valid A record (IE MX record: mail.yourdomain.com and A record: mail.yourdomain.com points to 123.123.123.123

2) Check your mail configurations: Most SMTP servers are not configured as open servers, as a spammer could use your mail server to send their mails. Look at 'mynetworks' or allowed hosts and make sure whoever is sending mail to your server is in this list.

3) Check the logs: Turn on verbose logging for your MTA. With postfix, edit /etc/postfix/master.cf and append " -v" at the end of smtpd (smtpd -v) and by default is logged to /var/log/messages

As for reverse DNS, just make sure that the hostname of your mail server is a FQDM (Fully Qualified Domain Name) and has the appropriate A records assigned to it. For ex. if your server's public address is at 123.123.123.123, and the hostname of your machine is mail.mydomain.com, then your A record should be mail.mydomain.com pointing to 123.123.123.123. in /etc/hosts you should have an entry with the same name (IE 127.0.0.1 mail.mydomain.com localhost) or (123.123.123.123 mail.mydomain.com)

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Mx record / A record need to be register in dns? It is not for home user? –  Leo Chan Mar 21 '12 at 17:02
    
Thanks for the informative answer –  Leo Chan Mar 21 '12 at 17:12
    
Correct. You can set up an SMTP server at home, too but the methods I described are for a fully public mail server. If you are simply setting up SMTP at home, I advise you try CJC's suggestion to see if you are actually able to send mail from your internet connection bypassing ISP server settings (Most providers will block port 25 for SMTP unless you specifically send mail to their servers) –  Ben Ashton Mar 21 '12 at 17:42

OK, if you're setting up a mail server at home, it'll be best if it's set up as a relay to either your ISP's mail server or to a mail server you can authenticate with.

Most likely, you will need to set up authentication, so you can SMTP AUTH against the other mail server. This sort of configuration is called a "smart host". Here's Wikipedia on the idea:

http://en.wikipedia.org/wiki/Smart_host

How you would set this up depends on what mail server you're using. Here's the bit from Postfix's docs: http://www.postfix.org/postconf.5.html#relayhost

There should also be documentation on how to configure authentication in Postfix.

You will also not use port 25. Most likely, you'll connect on the SMTP submission port, 587, or SMTP SSL, port 465. ISPs will not block those ports.

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