GMX is a file extension that is used by an email program to send group messages called GroupMail. GMX is a file that is used to store a message created in the program. Such messages can be created on the basis of templates that are pre-installed in the program or created by the user himself from scratch. As GroupMail is based on the WYSIWYG interface, no HTML knowledge or web design skills are required from the user.

GroupMail is most often used to create and send messages in the form of newsletters, marketing content or newsletters.