Mailbird is built on the Microsoft .NET Framework, so we depend upon Microsoft to support each version of Windows as well. This means that we will support any versions of Windows which Microsoft itself is still providing support for. Each version of Windows has a product lifecycle, beyond which Microsoft will no longer offer support

That lifecycle can be found here: https://support.microsoft.com/en-us/help/13853/windows-lifecycle-fact-sheet

Once Microsoft no longer supports a specific version, we may select to no longer support it as well, but since we want Mailbird to be compatible with as many versions of Windows as possible, we only drop support if absolutely necessary (for example, to stay up to date on important security updates in later versions of the .NET Framework without which our users may encounter security flaws or vulnerabilities).

For specifics on why Windows 7 and Windows 8.0 are no longer supported on the .NET Framework (which Mailbird is built on), I would suggest taking a look at this article: https://www.quora.com/Why-Windows-8-is-the-only-operating-system-which-is-not-supported-by-NET-Framework-4-7-so-software-based-on-it-too-Its-both-from-Microsoft