How to implement automatic sending of letters to the client after performing certain actions on the site?

Question

MODX

There is a site on modx. It is necessary that after the acceptance of the order and the beginning of its processing, a letter is automatically sent to the client stating that the order has been accepted for processing. After execution, a letter was sent, where several fields will change.
How to implement it and what class of systems does it? CMS, online user support service, CRM, mailers?