I think it is some sort of universal law of dating, that anyone who has been dating for any length of time must have a psycho ex.
My psycho ex dumped me by IM (I kid you not) and then was totally shocked and surprised that I moved on!
He honestly thought I would spend the rest of my college life sitting in my dorm room pining away for him.
A few months after he and I broke up, I met my husband, and a few months later we got engaged. Word got back to my ex, who immediately popped the question to his girlfriend at the time, then IM'd me to say "Congrats on your engagement, I'm so sorry I hurt you, will you ever forgive me, btw, I'm engaged too." (They later broke up.)
I think my ex just couldn't bear to see me so happy with the love of my life while he was still single. Sounds like that might be Jack's deal too. Well, it doesn't matter... just ignore his emails, block his IM's, and enjoy your future with Mr. Right.