First let's look at federal law. According to 18 U.S. Code § 2261A - Stalking:
Whoever... with the intent to kill, injure, harass, intimidate, or place under surveillance with intent to kill, injure, harass, or intimidate another person, uses... any... electronic communication system of interstate commerce... to engage in a course of conduct that—
(A) places that person in reasonable fear of the death of or serious bodily injury... or
(B) causes, attempts to cause, or would be reasonably expected to cause substantial emotional distress...
shall be punished as provided in section 2261(b) of this title.
"Course of conduct" is defined as at least 2 acts with a continuity of purpose, so the large amount of emails certainly qualifies. And email is obviously an "electronic communication system of interstate commerce". Then the question becomes whether he had intent to harass or intimidate you, and whether his conduct caused (or attempted to cause, or would be reasonably expected to cause) substantial emotional distress.
The federal laws against email spam seem be be directed against commercial emails, and don't seem to be useful here.
Next we can look at Massachusetts law. Part IV, Title I, Chapter 265, Section 43 deals with stalking:
Whoever (1) willfully and maliciously engages in a knowing pattern of conduct or series of acts over a period of time directed at a specific person which seriously alarms or annoys that person and would cause a reasonable person to suffer substantial emotional distress, and (2) makes a threat with the intent to place the person in imminent fear of death or bodily injury, shall be guilty of the crime of stalking...
Unless he made a threat, this doesn't seem to apply.
Section 43A deals with harassment:
Whoever willfully and maliciously engages in a knowing pattern of conduct or series of acts over a period of time directed at a specific person, which seriously alarms that person and would cause a reasonable person to suffer substantial emotional distress, shall be guilty of the crime of criminal harassment...
He's definitely engaging in a "pattern of conduct" "directed at a specific person". The question then is whether he's doing it "maliciously", whether it "seriously alarms" you, and whether it "would cause a reasonable person to suffer substantial emotional distress".
That's what the law says. I can't say I know whether what he did crossed the line - I don't know the content of the emails and what your emotional state is.