Email client, fork of Nylas Mail, it provides a high configurable client.
Free Software/Open Source (GPLv3)
needs a replace/obsoletes migration path
Would this still be the case given that it is a fork of an unmaintained project and could now be considered a separate project? Not that I mind having old orphaned packages removed from my system but it certainly feels better to have an option in the software center to consent to the removal of orphaned packages in case there is no replace migration path.
Today they released a .deb and a .rpm for Mailspring. It seems that they will make a snap too, so maybe the best idea would be to collaborate with them in some way to accelerate that snap building :D