Mail app opens automatically at startup

For the past few weeks, the apple Mail program has been opening automatically upon starting my computer or login. I've never set this as a preference and have not found anywhere to turn it off in either the Mail or system preferences. I have also checked startup items and it is not listed there, either. Normally this wouldn't be a problem, but when Mail starts up automatically like this, sometimes the mailboxes seem to have problems loading and I have to restart the program to fix them. This automatic startup began spontaneously, so I'm assuming it's a result of a software update. Has anyone else had this problem and does anyone know how to turn the automatic startup feature off? Also, this has been happening ONLY on my 15" MacBook Pro and NOT my iMac 24", even though they both have OSX 10.5 Leopard. Thanks in advance for any help.