Mozilla's Thunderbird email client is expected to get support for notifications natively in Windows 10, meaning that they will be logged into the Action Center for later viewing should they appear while the user is away. This also gives the user greater control over these notifications as well, as native support in Windows 10 also means they can be disabled automatically when Focus Assist is turned on.
Thunderbird is Mozilla's email client. It can serve as a great alternative to the Mail application pre-installed on Windows 10 devices, especially as it uses Mozilla's reputable Gecko layout engine, which renders emails much better than what Word does in Mail.
Mozilla hasn't given an timeline of when support for the Action Center be available, but the organization is also working on other ways to improve the email client, such as enhancing support for Gmail, overhauling UI elements such the application settings, and improving email encryption. Perhaps with enough improvement, Thunderbird could potentially regain the attention it once had before Google popularized webmail.