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ISP is blocking port 25

First I signed up for a free DNS hosting account through Mydomain.com I registered my domain through them years ago. So I have control over my own DNS records.

Second I went to NO-IP.com and signed up for the $39.99 mail reflector account. I found a great article by them on how to set it up here :


It was quick and easy. I paid my $39.99 and added my domain name. I put in port 2525. Then I added my email alias in the place provided by their site so when they receive my mail it would not bounce.

Third my DNS MX(mail exchange) record pointed to my domain ip address, so I had to change my my record to point to the no-ip.com mail servers. I changed my MX records to mail1.no-ip.com and a secondry mx record to point to mail2.no-ip.com.

Fourth I logged in to my SME server. I went to port forwarding and set port 2525 to forward to port 25 with the ip set to the word localhost.

It worked perfectly. Here is my litte map:

Incoming Mail

Yahoo mail --> MX record check --> Mail1.no-ip.com (port25) --> My SME server (port 2525).

Outgoing Mail

My SME server ---> My ISP's SMTP server ---> World

Works perfectly.

Important.png Note:
By using this approach your SME Server thinks that anything coming in from the outside world via port 2525 is in actual fact coming from within your LAN on port 25, so there is much less security applied.

Alternative iptables Approach

I've added the following lines to /etc/rc.d/rc.local

iptables -A PREROUTING -t nat -p tcp --dport 9993 -j REDIRECT --to-port 993
iptables -A PREROUTING -t nat -p tcp --dport 4465 -j REDIRECT --to-port 465

Cut-paste example for port 2525 redirection:

iptables -A PREROUTING -t nat -p tcp --dport 2525 -j REDIRECT --to-port 25

Alternative iptables Approach

ISP is blocking port 80

Same deal except different ports