My understanding is that e=mc2 is for objects not in motion.

The full formula for all objects is said to be :

e2=(mc2)2 + (pc)2

Where p is the momentum of the object in question and c is still the speed of light like in the mc2 part.

A great visual of the concept using a right angled triangle can be found at the following link:

https://www.britannica.com/video/185388/equation-theory-energy-relativity-mc