These days receiving mail is relatively rare, unless you mean email, so what could make it even more special? Gorgeous Calligraphy! I sent out thank you cards to my vendors and hired the services of Lesley at Letter Be On Etsy to add that totally amazing touch of class that comes with handwritten calligraphy addressing. Sshe did gorgeous white addressing on red envelopes (supplied by me) with opaque white ink. I especially love how the address goes off the edge of the envelope!
Red envelopes look amazing for Christmas Cards - you can find them on Amazon - I recommend 70lb paper because it's thicker and feels more luxurious - like these: 100 Red A2 Envelopes - at the time of this writing they're around $10 for 100 which is a great deal!
Combine them with some cool stamps, and your letter recipients will be in love! I wrote a post a couple years ago about where to find great postage: Where to buy Vintage Stamps Online