I am not immediately able to repro.
I tested with https://try.discourse.org which has its min post length site setting set to the default of 20.
I created a new topic with User A, then replied to it with User B. User A received an email and I sent a one word “Test” reply via email:
I received an email right after with an appropriate message:
@adrelanos are you able to provide any additional reproduction steps? Is it possible that the message you received was valid somehow? Can you also confirm you are on the latest version of Discourse?