This is an 'academic' type letter in which Seneca outlines the argument that virtue is a body or corporeal. There is nothing new or significant, in my opinion, about this letter. For those interested (who are reading this letter), I would direct you to a couple of links.
See my summary of John Sellars chapter on Stoic Physics. It discusses this at a good level as to what the argument was and why the Stoics took the position they did.
The Internet Encyclopedia of Philosophy and the entry on Stoic Physics is good too.
One other thing I will note from this letter - his introduction. He writes,
My tardiness in answering your letter was not due to press of business. Do not listen to that sort of excuse; I am at liberty, and so is anyone else who wishes to be at liberty. No man is at the mercy of affairs. He gets entangled in them of his own accord, and then flatters himself that being busy is a proof of happiness.
This is an important observation he makes. How often do we make excuses for things; and blame the excuse on something beyond our control, when in fact, it was up to us? Indeed, blame 'busyness' for why you did not do something, but you were still 'at liberty' to stop the busyness and do whatever might be more important.
It may seem like a minor quibble, but it's worth a self-reflection. Our choices make us who we are. The tangled web of events are complicated, but be sure to not entangle what is 'up to you' in things that are 'not up to you.' Take responsibility for what is 'up to you' and be precise when casting blame to a certain cause.