PopOS 19.10 (Ubuntu 19.10)
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I have used Mailspring on 18.04 previously. So I do have an account to login with. After going through the initial setup screen I click sign in and get the following error:

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I am using mailspring on Ubuntu 19.10 as well and didn't encounter the error you are experiencing.
Did you try to install potential missing libscret and related libs? https://www.howtoinstall.co/en/ubuntu/xenial/libsecret-tools
@mhatvan - I can install this manually. However, I feel if MailSpring depends on this lib it should be added to the dependencies. Otherwise MailSpring will not work on distros that are built on the Ubuntu base image. (Ubuntu based distros)
@mhatvan - I have removed MailSpring for now. I'll wait until they fix the dependencies before I try it again.