Is there a way to download and install BlueMail from https://bluemail.me/? They do not have a flatpak that can be downloaded from Flathub.
It’s closed source. Maybe they have a tarball version or something else - you should ask the makers of this software.
I have asked. They said they would make a flatpak available “soon” but I wanted to know if there was a way to install it prior to that.