Why would I have got exim4 installed on one system but not on another?

[Mario Vukelic]

On Sun, 2009-01-25 at 13:27 +0000, Chris G wrote:
> I have three xubuntu 8.10 systems installed here, one has exim4 on it,
> the other two don't - they appear to have no mail delivery system on
> them at all.

Ubuntu packages should in general not depend on a particular MTA, but on
the virtual package mail-transport-agent, which is provided by the
following packages: citadel-mta, courier-mta, esmtp-run,
exim4-daemon-heavy, exim4-daemon-light, masqmail, msmtp-mta, nbsmtp,
nullmailer, postfix, sendmail-bin, ssmtp, xmail.
Only if a general MTA is not sufficient, but the package really requires
as specific one should it depend on this MTA directly.

However, some packages (probably from the universe repo) still
incorrectly install exim4 if you have no package installed that provides
mail-transport-agent installed (which is the Ubuntu default).

You can find out what packages depend on exim 4 by "apt-cache rdepends
exim4". It's quite a list.