We have the latest Greenshot 126.96.36.199 installed on Windows 10 Enterprise 1809 with Office 2019 Professional Plus installed. However, Greenshot does not show Outlook as an available destination.
We tried entering the Outlook program as a custom external destination command with the standard  for passing the screenshot name, however Outlook says that the command argument is invalid.
The send to Outlook feature is working fine with Outlook 2016. My assumption as a technician is that Office 2019 / O365 now installs with the new Microsoft "click to run" technology instead of the older MSI technology. I have seen other instances of software that are not able to "see" that Outlook 2019 / O365 is installed because of different registry keys associated with the click-to-run install.
We installed Office 2019 with the free Microsoft "ODT" tool.
Office 2019 is rare, however it is just an earlier version fork of O365.
We are a healthcare clinic and our managers would love to have this feature returned on their newer computers.
Windows 10 1809 Enterprise, Office 2019 Professional Plus Enterprise (click-to-run install)