Not receiving email notifications of iCal event invitations from other parties.

I am not receiving emails for iCal event invitations from other parties, but the invitations ARE in my iCal Inbox. I don't use iCal often, so I never proactively go in to check my Inbox there, but rather have always depended on receiving the email notification of an event invite to know to add it to my iCal. I can SEND invitations to others successfully with other parties receiving MY email notifications, I'm just not receiving them via email.

I've now missed two very important meetings because I never received the email invitations to add an event, which were the only forms of correspondence for both. Can anyone help me? Thank you!