Moving from Sendmail to Exim as MTA on CentOS

I recently began a move from our Solaris 10 based mail and file server to a new one running CentOS 5 x64. We are running Exim as our MTA instead of Sendmail mostly because it is a lot easier to set up and it is at least as stable as Sendmail and furthermore the command line is backwards compatible with that of Sendmail.

On Solaris I always compiled the source myself but now I'm trying to skip this practice on CentOS in favor of 'yum' and thereby getting easier updates and faster software installation. As a package newcomer there is a bit of a learning curve and I think I loose a bit of the knowledge on what is happening behind the scenes when I'm not compiling the stuff myself (did the package makers really do it the way I prefer?).

Well, Exim is really a bit old school when it comes to source configuration. It is not autoconf compatible and it has it's own scheme where you manually edit a configuration file before you make the source code. Also things like user and group id's are hard coded into the executable as well as log file paths. That's one of my main Exim complaints...

When installing Exim on CentOS I first created a exim username as well as an exim group. I chose 90 as id for both as it was my preferred Solaris default. You are not able to install Exim unless the user and group exists but the installer does not check if it is actually the id of the hard coded value in the executable.

So when starting Exim I got a lot of errors like this one:

maillog:Jan 22 11:57:51 osiris exim[20345]: 2010-01-22 11:57:51 cannot run initgroups(): no passwd entry for uid=93
maillog:Jan 22 11:57:51 osiris exim[20345]: 2010-01-22 11:57:51 Cannot open main log file "/var/log/exim/main.log": No such file or directory: euid=0 egid=93
maillog:Jan 22 11:57:51 osiris exim[20345]: exim: could not open panic log - aborting: see message(s) above

From that I could conclude that uid and gid was expected to be 93 and after a change to the passwd and group file the program started nicely.

So the directions is as following:

Add to /etc/passwd:

exim:x:93:93:Exim Mail Transport Agent:/:/sbin/nologin

Add to /etc/shadow:

exim:*:::::::

Add to /etc/group:

exim:x:93:mail,exim

And to be safe add exim to the mail group:

mail:x:12:mail,exim

Then you install Exim via yum:

yum install exim exim-doc exim-mon

Copy your old configuration to /etc/exim/exim.conf and add exim to rc.d, disable sendmail and start exim:

chkconfig --add exim
chkconfig sendmail off

service sendmail stop
service exim start

Now you should be more or less up and running. Remember to configure your firewall and MX record or spam filter to point to the new mail server.