1. does the card get sent?
No, 4images check if the recipient address is valid and then sends email, otherwise it will show u an error message.
2. does 4images code make a return email required?
not sure what do u mean by that.
3. will the return email address be that of my server (since the sender didn't specify one)?
if u specify wrong email address that is not exists, the return email will be send to email u specifyed as FROM (actualy its depence on how email server configured).