The .msg file format is native to the Microsoft ecosystem. It is related to the Microsoft Outlook email program and the Microsoft Exchange server, and stores email messages, calendar entries, appointments, etc.
Microsoft publishes a lengthy specification of this format at the following address, if you are interested:https://docs.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-oxmsg/b046868c-9fbf-41ae-9ffb-8de2bd4eec82
The easiest way to create such a file is to drag a message from Outlook to a folder, or to the desktop.
It gets more complicated if you need to view such a file, but you don’t have Outlook installed. There are a few viewers, but they are paid by default. There are virtually no free or easily accessible options to view .msg files on Windows, Mac OS, and Linux/Ubuntu. At least, on Linux you can install a Perl script, called msgconvert, which converts .msg files to .eml via terminal commands. However, not all people are comfortable working with text commands. There are a few online converters, but the actual viewers are scarce.
MsgEml.com tries to fill the void and allow viewing .msg files on Windows, Mac OS, and Linux, without having to install any software.