[CentOS] Apache wakes-up inactive Exim

Had a surprising event on C 6.5.

Exim was the only MTA installed. It was partially configured (with ACL,
Router, Transport) and definitely not running.

I was remotely testing a web page. A web page error condition invoked
the embedded PHP mail() command.

To my astonishment something in Centos woke-up Exim. Exim sent the email
and then became inactive again. The Exim logs does not show any start-up
lines, just 

1. input from Apache.
2. output to remote server.
3. 'completed'.

Hours later Logwatch, not yet customised, also caused inactive Exim to
send an email (which got rejected by Exim because it was to local user
0).

What causes Centos to temporarily activate in-active (meaning
non-running) Exim ?

Thanks,

Paul.
England EU.