By default Mac OS X (non-server version) does not start Postfix.

To start it (and have it auto start), do the following:

$ sudo vi /System/Library/LaunchDaemons/org.postfix.master.plist

Add the following before the </dict> tag:

<key>RunAtLoad</key>
<true/>
<key>OnDemand</key>
<false/>

Make sure the the plist is loaded:
$ sudo launchctl load -w /System/Library/LaunchDaemons/org.postfix.master.plist

Then start postfix with:
$ sudo launchctl start org.postfix.master

After that, it should start automatically on reboot. For better deliver-ability (such as a real domain instead of .local) you'd need to do a standard postfix configuration change.

One issue with this "phone home" method, however, is that if port 25 is blocked (which many ISP's do), the mail won't go anywhere until the mac is connected somewhere unblocked. You might be better off (and save the trouble of above) by setting a cron job to periodically hit a webserver you control, such as http://yoursite/mymaclocation and then you'll continually have a log of where your mac is, and port 80 is rarely blocked.