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?

שליחת הודעה לאחר פרסום

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

You could display a banner for a given category

נושא		תגובות	צפיות
Show customisable message on private topics/categories? Support	5	1150	16 בספטמבר,‏ 2021
Display banner topic only to logged in users? Support	3	1302	17 באפריל,‏ 2019
Post or Topic Visibility Support	6	521	8 בנובמבר,‏ 2023
Showing two banners: one to anonymous users, and one for logged-in members Support	5	1326	4 במאי,‏ 2019
"This topic is only visible to those who can see the {x} category" message UX	3	598	16 ביולי,‏ 2020