Why does Apple Mail keep opening?

I used to use Apple's Mail program in the past but because of a lot of needs I have with many email addresses I switched to another program, Mozilla Thunderbird. I have used it for a couple years and I never open Apple Mail.
Approximately a month ago Apple Mail started opening. I don't quite know for sure when it opens but I come to my computer to see it open but have never opened it and I simply close it to find it open the next day. I think it may have happened after one of the OS X updates but not the latest 10.6.8

I know it's not a computer specific issue since I have 3 Mac's and every one of them is doing this. Does anyone know what could be causing OS X to open Apple Mail? Is there a way I can stop it from doing this? I don't want to remove the Apple Mail program since it may cause additional issues.

Thank you for your help.

You are wise in knowing better than to try and remove Mail. Pat yourself on the back on that one.

There are two things I can think of that would cause Mail to open without you doing it:

1. Someone has clicked on the "open at login" selection from the Mail icon in the Dock. You can re-select that to UNcheck it if that is the culprit (just click and hold on the Mail icon in the Dock, you'll see "options")

2. Mail is still selected as the "default" mail client. To fix this, you must open Mail, go into the first pane of the preferences panel, and point it to the email client you actually use.