Hi Heytherepumpkin,
A good response is, “Sorry you feel that way!”
If this is an ongoing conflict/issue, and ironically it is all about him, write, “There’s no reason to write that to me, you can write to me respectfully. Try again.”
Or, write (again), “Are you OK?” if he is unhinged. Or the classic, “Have you been drinking?”
Good Luck,
Inky