Archived - iCal meeting invitations are sent using default Mail account
When sending a meeting invitation by email using iCal, Mail uses the email account which is currently listed in the contact you've created for yourself in the Address Book application.
If you do not have a contact established for yourself in Address Book, Mail will use the default email account that's been configured to send iCal invitations.
iCal uses the Mail application's default email account if you don't have a different (or additional) account listed in your own contact entry in Address Book.
You can specify which account is the default in Mail preferences.