GMime is a C/C++ library for parsing and creating messages using the Multipurpose Internet Mail Extension (MIME). This is GMime-3.0 which installs in parallel with GMime-2.6 (port gmime).
To install gmime3, run the following command in macOS terminal (Applications->Utilities->Terminal)
sudo port install gmime3
To see what files were installed by gmime3, run:
port contents gmime3
To later upgrade gmime3, run:
sudo port selfupdate && sudo port upgrade gmime3
Reporting an issue on MacPorts Trac
The MacPorts Project uses a system called Trac to file tickets to report bugs and enhancement requests.
Though anyone may search Trac for tickets, you must have a GitHub account in order to login to Trac to create tickets.