Hi @Ondrej_Sonka , welcome to the OpenEMR community! I hope you can find all the answers you need here.
The OpenEMR wiki is poorly maintained as far as removing out- of- date links and the one you show about Mercury Mail is one that should be deleted. Mercury Mail has been obsoleted in OpenEMR for production use with real PII because of its poor security. It is still used for demo purposes or to get email- based services configured before switching to a secure SMTP provider. But then, plenty of modern free ones are available so you might as well just start with the secure ones. You have not mentioned what use you are putting Mercury Mail to; unfortunately, this might mean that all your work configuring Mercury Mail might need to be re-done with another service?
Here are a few current links re: setting up SMTP servers and email communications through OpenEMR. Patient self- registration in the portal is one of the most common uses for the feature so you will find several pages relating to that.