How to setup a trivial forwarding mail server on a VPS

05 Nov 2008

Switching to a virtual private server (VPS) from shared webhosting is a great way to improve performance, stability and security. For the most part, setting up the webserver and database is fairly routine and straightforward. However, setting up an email server is non-trivial.

It’s a lot of work to run a full mailserver (and which is why some webhosting companies recommend that you use a dedicated email service) but there are ways to setup a trivial mailserver that requires minimal maintenance. It simply accepts mail to various domains and then forwards them on to an actual mailserver.

For this example, we’ll have two domains running on our VPS: xxx.com and yyy.com

We’ll also have an “info” address at each domain that will be forwarded to xxx@gmail.com and yyy@yahoo.com

First, install a mailserver. I use the CentOS distribution which has the yum package manager. It’s as easy as:

$ yum install exim

We need to create a file for each domain that we’ll be servicing.

Create a directory to hold our virtual hosts:

$ mkdir /etc/exim/vhosts

Create a configuration file for each domain. Use the same format as the /etc/aliases format.

# file:/etc/exim/vhosts/xxx.com
info: xxx@gmail.com

# file:/etc/exim/vhosts/yyy.com
info: yyy@yahoo.com

We need to tell exim to use these files when delivering mail. Near the top of /etc/exim.conf there will be a domainlist line.

# change...
domainlist local_domains = @ : localhost : localhost.localdomain
# to...
domainlist local_domains = @ : localhost : localhost.localdomain : dsearch;/etc/exim/vhosts

Now exim will accept mail for our domains. To have it forward the mail correctly, add the following “router”. Look for begin routers. The first router is probably “dnslookup”. After this, add the following:

virtual:
  driver = redirect
  domains = dsearch;/etc/exim/vhosts
  data = ${lookup{$local_part}lsearch{/etc/exim/vhosts/$domain}}
  no_more

Restart (or start) exim to have these changes take effect.

$ /etc/init.d/exim restart

MX Records for your domains

You have have a trivial mailserver. However, you need to specify that your server handles the mail for your domains. This is defined in the MX (Mail eXchange) record. To view the current setting:

$ dig -t mx xxx.com

If the wrong address is returned (or no address) then you need to update your domain name servers. (Whoever provides you with domain name servers - typically your VPS provider or domain registrar - they should be able to set up MX records for you, or provide you with a tool so you can do this yourself.)

Testing and Debugging

If you view the exim log file, you can watch mail being processed.

$ tail -f /var/log/exim/main.log

If mail is not being routed directly, or you want to see more info, the debugging feature of exim is helpful. You can activate this by passing in ‘-v’ flag when starting exim.