Apple Mail keeps Opening - Why?

I finally just renamed Mail.app as Mail.zip and that's put a stop to it. However, anytime my calendar (iCal) alerts me that something is coming up, Mail started opening.

I absolutely detest the half baked integration of Apple Mail with Gmail's IMAP, so many bugs it's unusable, so I've never had it setup as my default mail program. I've used Thunderbird and now Postbox.

Can anyone explain to me how/why mail keeps opening up on me? It's probably something simple but I can't find it.

If it helps anyone else:

I couldn't find an alert or any other reason for Mail to keep opening. I also do not use Apple Mail for my email, I use Postbox (postbox-inc.com) so I created a backup of the mail program and deleted it from my Mac. Now it doesn't open any more.