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

[Chris G]

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.

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

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

-- 
Chris Green