Lactating mare

Question
This is our first mare, and I was grooming her the other day and noticed thick black stuff kinda like the stuff when you clean a geldings sheath in between her teets.  When I was getting it off I was squeezing her nipple and a milk like substance came out it look kinda like skim milk is this normal? Should I be worried? Our other mare is pregnant and she doesn't even have a discharge.  Their bags both hang down about the same amount.  The mare with the discharge shouldn't be pregnant, and doesn't look pregnant.  Do mares do this when their in heat? She use to be a broodmare we just got her a week ago, could she have already had a foal and it was weaned and she hasn't dried up yet.  

Answer
Hello!
I think i know exactly what you are talking about. The black stuff between her teets, is acalled wax. All mares have some, but pregnat mares or mares that just foaled have more. If milk substance came out of her nipples, I would say it is because she foaled, and weaned the baby not very long ago. I am 99% sure that that is what it is. I wouldn't worry about it. It is completely normal.
Good luck
Thank you for the question!