Is it safe to reheat tea with milk?

The official answer from your health department is no, it's not safe. After four hours at room temperature, you have to pitch it.

That standard is a little on the paranoid side. Which is appropriate, since they have to protect people for whom a little salmonella is potentially life-threatening (the infirm, babies, the elderly, etc.) If you're young and healthy, the odds are it will do you no harm whatsoever. Even a glass of milk out overnight is merely going to be sour rather than toxic.

So, it's up to you. I'd say it's a cup of tea and it will taste better if you make a fresh one, even if the risk to your health is small-but-not-zero.