Hi! I want to make sure that users know about a certain rule before posting in a certain category. Can I show a message to users before or after posting in this category and/or send an automated message via Dicobot or Email?

So you want those creating topics in that category to have some guidance? Have you seen What are Topic Templates?

Sending a message upon post

isaac (Isaac Janzen) September 19, 2022, 4:39pm 2

You could display a banner for a given category

Topic		Replies	Views
Show customisable message on private topics/categories? Support	5	1146	September 16, 2021
Display banner topic only to logged in users? Support	3	1296	April 17, 2019
Post or Topic Visibility Support	6	511	November 8, 2023
Showing two banners: one to anonymous users, and one for logged-in members Support	5	1312	May 4, 2019
"This topic is only visible to those who can see the {x} category" message UX	3	594	July 16, 2020