Where is this cron job running from?

On my Debian system, /var/log/syslog contains an entry for each cron job which is executed …

Jan 17 20:10:01 machine1 /USR/SBIN/CRON[1545]: (convict) CMD (fetchmail -s >/dev/null 2>&1)
Jan 17 20:17:01 machine1 /USR/SBIN/CRON[1800]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Jan 17 20:20:01 machine1 /USR/SBIN/CRON[1914]: (convict) CMD (fetchmail -s >/dev/null 2>&1)

If your job is being executed by cron, you should find a reference to it in /var/log/syslog.

I have some things you can try that might help.

To see the cron jobs on the system, try this:

crontab -l

This is what I get (as I only have one job setup to run):

#CrInfo osascript -e "tell application "iTunes" to updateAllPodcasts"
30  2   *   *   *   osascript -e "tell application "iTunes" to updateAllPodcasts"

That will list the jobs that will run on a scheduled basis.

To list all the running processes on the system and their full filesystem paths type:

ps -xa

You'll get some output like this: (which should make it easier to know where to delete the script that runs your email)

 4968 ??         0:03.24 /Applications/Utilities/Terminal.app/Contents/MacOS/Terminal -psn_0_655520
 6112 ??         0:23.31 /Applications/QuickTime Player.app/Contents/MacOS/QuickTime Player -psn_0_712878
 7203 ??         4:33.38 /Applications/Transmit.app/Contents/MacOS/Transmit -psn_0_946407
 7240 ??         2:56.99 /Applications/Google Chrome.app/Contents/MacOS/Google Chrome -psn_0_975086
 7358 ??         0:00.02 /System/Library/PrivateFrameworks/KerberosHelper.framework/Resources/LKDCHelper
 7369 ??         0:04.72 /usr/sbin/AppleFileServer

There's also a page on IBM's site that might help you out with how to use Cron properly.

That said if your job doesn't appear in cron are you sure you aren't running it with some other tool that you've forgotten about? Some custom deamon application that you may have downloaded for example?