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

[Rashkae]

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.
> 
> I just discovered this because some cron jobs were failing on one of
> the mail-less systems and, not surprisingly, I didn't get an E-Mail
> telling me of the failure.
> 
> Surely basic (local) E-Mail delivery is an absolute necessity.

I would think so myself, but I'm old fashioned.. Ubuntu does not install
one by default, and the typical desktop environment works just fine without


> 
> Would the system which has exim4 installed have got it as part of a
> LAMP install?
> 

Try removing exim and see what packages it complains will also have to
go for dependencies


> ... and a final question, who/what decides that exim4 is the default
> mail server (if it is).
> 

The package you installed that required local mail delivery probably had
exim4 as it's first choice.  Exim is the default in Debian, so that
preference could have been carried over by a Debian package.