The system tries to find the best "From" email address starting with:
a) the Event Contact email address
b) then the Organizational Contact email address under Account > Account Details.
c) then the Calendar Administrator which is a no reply address.
When you add or edit an event, if you enter a contact email in the event contact section of the Description and Details tab that email address will be used. If that is not present then it will use the Organizational Contact email address under Admin Menu -> Account.
Finally the system will use the generic address in option c above if neither option a or b is available.