Why should this be included in the repository?
It is a very nice electron based email client. It offers some advanced options like link tracking and send later.
Is it Open Source: yes
How many users do you anticipate will use this software? I believe anyone who uses a desktop email client will want to use this. In my personal preference, is more fun to use than Thunderbird.
Link to source tarball/zip file: https://github.com/Foundry376/Mailspring/archive/1.0.7.tar.gz