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.
Pasteurized milk is nice in that it spoils to our nose and palate far before it becomes unsafe. So if the tea still smells and tastes fine, it is safe to drink, assuming that your milk is pasteurized.
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.