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 10.6.8I 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.