I love sending Christmas cards (about 70) each with an individual personal letter (not a round robin - I hate those). And the envelopes are decorated with Christmas stickers, (one per envelope) and always the return address on the back.
I cannot stress enough the importance of a return address on the back of the envelope. Many a misunderstanding has been caused because the recipient has either died or moved and the new occupant has no way of letting you know - a card simply signed "with love from Gran" doesn't give anything away. At the very least, Royal Mail will return it to you with a sticker letting you know why it cannot be delivered (assuming the recipient puts it back in the post box marked "unknown" ) . Even if it means you no longer have contact, at least you know not to send to that address again.
I worked for Royal Mail for 20 years and one of my duties was to send piles of returned Christmas cards (and gifts) to the Dead Letter Office because there was no return address on the back. The DLO open these letters to try and find a return address inside, in which case they will reseal it and return it, but 90% of these letters are just loving Christmas cards, signed with good wishes and no other clue who they're from. These are destroyed. Here endeth the Royal Mail lecture, but if you have any questions, I would be happy to answer.