I have leftover tea from last night but it has milk in it. Is it safe to reheat it the next day and drink it? Or should I just bin it?
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.
I generally don't recommend it, but when you store the tea in the refrigerator it should be safe. Make sure to cover the cup with a lid, because it's easy for the tea to absorb flavours in the refrigerator. If you've meat, fish etc...in your fridge you're not gonna love it.
A tea bag (or even loose tea) isn't that expensive anyway, when prepared at home, so the best would be to just enjoy fresh tea.