It's absurd that a Mailspring is required in order to use Mailspring as an e-mail client. It's not even possible to see the UI in order to determine if the app is worthwhile before creating said account.
There are dozens of reasons why a Mailspring account is useful. There are zero reasons why it is necessary. As such, making it necessary before the installer even runs is either a bug or a design flaw, and it should be fixed.
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Agreed, you shouldn't need a mailspring account to use the client. What if you have an outage, how many 9s of uptime does my mail client have?