The FreeBSD Diary -- Is your ISP blocking port 25? Here's a Postfix solution.

Mon Jun 11 14:08:38 UTC 2007

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Is your ISP blocking port 25? Here's a Postfix solution.10 February 2006
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My ISP started blocking incoming port 25. It's already blocking  
outgoing port 25 and I'm handling that. Now it's time to start  
accepting incoming mail on the submission port, 587. They aren't  
blocking my incoming port 25. But we went through this process for  
another guy on our computer, so I figured that this is a good thing  
for which it will pay to be pro active.

This solution assumes you have a mail server at home and at least one  
other mail server out there on the Internet, one which does not have  
port 25 blocked. That part is crucial to this solution. It is the  
external server[s] that will accept incoming mail and forward it to  
you. In DNS terms, your MX records will not point to your home  
server, but to your public server.

Your home mail server
I started by adding the following line to /usr/local/etc/postfix/ 
master.cf on my Postfix mail server at home:

10.34.0.1:587 inet n - n - - smtpd
where 10.34.0.1 is the public IP address of my mail server [no,  
that's not really my IP address]. This instructs Postfix to listen on  
that IP address on port 587. This is known as the submission port:
$ grep 587 /etc/services
submission      587/tcp
submission      587/udp
Your public mail server
Then I added this to /usr/local/etc/postfix/main.cf on my public mail  
server:

transport_maps = hash:/usr/local/etc/postfix-config/transport
This tells Postfix to observe the transport directives in the above  
mentioned file. You can put the file whereever you want. I like to  
keep it in that directory, which you'll probably have to create  
because it's not part of the standard system. In /usr/local/etc/ 
postfix-config/transport I have:

myserver.example.org    smtp:[myserver.example.org]:587
Where myserver.example.org is the hostname of my mail server at home.  
You need to create a .db file to go with that. I issued these commands:

cd /usr/local/etc/postfix-config
postmap transport
You should now see a transport.db file. After making these changes  
you should restart postfix:

postix restart
Testing
Then I sent a test message from the public mail server

$ echo 'test' | mail me at myserver.example.org
I confirmed that it was coming in on port 587 with this command on my  
mail server at home:

tcpdump -i fxp0 port 587
Where fxp0 is the outside NIC on my firewall (the one with IP  
10.34.0.1) as shown above.

Then, on the public mail server, I requeued all the messages, so  
they'd use the right transport:

postsuper -r ALL
It's magic!

All the messages were delivered to the right spot.

Controlling access
I control access to port 587 on my mail server. I have firewall rules  
in place that allow connections only from my home server. I think  
there are no security risks involved in keeping it open, but I see no  
reason to give access where no access is required.

What about the other way around?
If you need to handle outgoing port 25 to avoid ISP blocks, you can  
always the same instructions, but in the reverse direction. It should  
just work.

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