If you receive an event invitation (from an Exchange Server), Lightning lets you accept or decline it and offers to send an Event Invitation Reply. The problem is that the email account used for sending this reply is picked a bit differently than users would expect. The account can be set in the calendar's properties. However if a user gets invitations from multiple sources (via multiple email addresses) into the same calendar, you cannot tell it to reply using the same email account that received the invitation. You can set this only on a per calendar basis. What a pity.