How To Relay Email On A Postfix Server

Introduction
For two small businesses I set up a debian lenny installation on their "home" dsl connection. The problem is that they have dynamic ip addresses and most mailservers will not accept incoming mail from a server on a dynamic ip address. The solution is rather simple. Set up postfix in a way that it will relay the outgoing email through the actual ISP. In this short howto I'll show you how to do that.

1. Prerequisites
I assume that you already have set up a working postfix server and that you have an email account at your ISP which you can access. So you will need to have a login for your IPS's email account.


2. Edit the postfix config
First you need to edit your postfix config...

nano /etc/postfix/main.cf

... and add the following code at the end of your config:

smtp_sasl_auth_enable = yes
smtp_sasl_security_options = noanonymous
smtp_sasl_password_maps = hash:/etc/postfix/saslpasswd
smtp_always_send_ehlo = yes
relayhost = smtp.yourisp.com
Of course replace smtp.yourisp.com with the actual smtp server of your ISP. Also SASL must be working. If you followed the perfect howtos for setting up a server as provided by Falko then you don't have to worry about it. Then save and close the file.

If your ISP requires you to use a special port for sending email, then use a line like this instead:

relayhost = [smtp.yourisp.com]:PORT
In one of the cases that I have used this, I had to enter this:

relayhost = [smtpauth.bluewin.ch]:587


3. Edit /etc/postfix/saslpasswd
After having extended the postfix config you'll still need to add the credentials to the /etc/postfix/saslpasswd file, so that you can authorize yourself at your ISP.

nano /etc/postfix/saslpasswd

and then add this:

smtp.yourisp.com yourlogin:yourpassword
Of course replace yourlogin / yourpassword with the actual username and password provided by your ISP. You don't need to add the port there.



4. Hash /etc/postfix/saslpasswd
Before postfix can use that file, it needs to be hashed by postmap:

postmap /etc/postfix/saslpasswd

5. Restart postfix

Finally you need to restart postfix to use the new config:

/etc/init.d/postfix restart