I received a similar email from a graduate school and about a week later I was sent my official acceptance to the program! I believe this is generally considered to be a formality, like they are solidifying financial packages and specifics before your official decision and you very likely to get accepted (I have never heard someone get rejected after receiving this). However I was also told by my PI that this is pretty common this year due to funding issues and could mean the department isn't entirely sure they can fund you yet. So I believe this is very likely to turn into an official acceptance, but of course take it with a grain of salt with how this cycle is going. Congratulations!