Atalose describes my experience to a T. Even now years later, I'm still getting angry delusional and drunk communications from ex, though much less than I used to.
For me, I had to figure out that I was responding much more than I needed to. 90% of what my ex sent me did not require any reply - it was him building up his narrative of victimhood or trying to bait me into an argument. And it worked a lot of the time, because I'm somewhat argumentative by nature and because it took me a while to lose the desire to prove to him that he was totally WRONG. You can imagine how effective I was in doing that.
I had (still have) a three-part rule for responding to any communication from ex. I ask:
1. Is it polite? (no name-calling, no swearing)
2. Is it about Kid? (because there's really nothing else to discuss with him)
3. Is it a direct question, a legitimate request for information about Kid, or providing me with new information about Kid that merits a brief acknowledgment?
If the answer to any of the questions above was "no", I didn't respond. This helped keep a lid on my tendency to want to "talk things out", which was just keeping me engaged with an angry alcoholic.
I found that dealing with him was a bit like training a dog - you reinforce the behavior you want to encourage (communicating politely and to-the-point) and ignore the behavior you want to extinguish (ten-page paranoid rants). Eventually the former increases and the latter decreases (although unfortunately does not completely disappear).