I understand the feelings. Neither T nor I celebrate Christmas, but on the holidays we share, I tell her good wishes at the session before the holiday.
It's hard to accept that we feel differently about our Ts than they feel about us.

On second thought, I don't think it would be wrong to send T an email saying "Merry Christmas", for those who are allowed to email, or would that be setting yourself up for disappointment if she didn't email back?

I send T a birthday card on her birthday, knowing I won't get one back.

Or is it the sense of intruding into T's personal time? Since I have trouble with boundaries, I'm wondering if you think that crosses a boundary.