The email address that you use to sign into your account is the address that is displayed to your recipients in the email’s ‘from’ and ‘reply to’ fields.
If a recipient clicks the ‘reply’ button in their email instead of using the RSVP form, then you would still be able to receive their message. Similarly, if you accidently sent an eCard or newsletter to an invalid email address, you would receive a ‘bounce back’ message, letting you know the card did not go through.