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	1132	September 16, 2021
Adding category description OR text at end of thread Support	7	39	September 15, 2024
Category Description Support	2	930	December 11, 2022
Display message for user when creating topic Support	4	721	January 28, 2024
Text info box in category header Support	12	2915	June 1, 2024