How do you handle reactions in Play by Post?

About to run a game here for a few friends. How would you go about handling reactions like 5e's Shield Spell, or Pathfinder's parry without getting bogged down in several posts?

my best guesstimate would be to run combat in blocks. Ally1 -> enemy1 -> ally 2 or whatever.
I don't think theres a real good way of going about it but I do believe that doing combat in blocks where each takes their turn and then at the end of that block you resolve all the actions (unless it slays an opponent then i would suggest to resolve that quicker then most) and just hope that the players are active enough to be able to post quickly if they wanted to react to something.

Another thing iv'e noticed too is that if a player does a thing that requires an opponent to save. to have them roll the d20 and add the necessary modifiers when you see it and apply conditions.