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	1075	September 16, 2021
Category Description support	2	727	December 11, 2022
Display message for user when creating topic support	4	674	January 28, 2024
Text info box in category header support	10	2817	April 22, 2020
Creating category where staff can see all post but users can only see there own support	5	2639	July 12, 2015