Almost all the pre defined emails that are sent to users in your Rocket.Chat server can be customized.
To customize an email content simply select the email you want to change, and set the Custom option to true.
From there you can change the subject and the body of the email. When writing the body of the email you can add placeholders that will be changed for real data when the email is sent. E.g. [Site_Name] will be changed to your Rocket.Chat server name.
You will have access to more placeholders depending on which email you are editing. For a detailed list of available placeholders check the text under the body field.