I just went to the download page and it says: "Mailpile packages are currently available for recent Debian-derived distributions, including Ubuntu. The packages are architecture-independent and should be compatible with most desktops, servers and embedded hardware (such as the Raspberry Pi). They have been tested on Debian 8, Debian 9 and Ubuntu 16.04 LTS."
So sounds like it will run on the Librem 5 at least.
So sounds like it will run on the Librem 5 at least.