One of my professors misbehaved with me. So after graduation, I wrote him a strong and humiliating mail. Can he get my degree revoked?
It is unlikely that he could do so, but it can't be ruled out universally. Basing a degree revocation on an email would seem to be beyond the pale and not legally supportable.
Don't however, make a point of publishing the email lest possible slander law comes in to play. If they want to retaliate against you and are unscrupulous, then they have other ways to do so by trying to affect your reputation. But that is independent of any email you sent.
Bad people behave badly.