Thunderbird has slowed down its development of features to only focus resources on security and stability. But still, Mozilla (the project owner) still accepts any improvements one submits as long as it follows their security standards.
If you have to manage many accounts, consider to waste a lot of time to configure them. If something goes wrong extra hours are required. Editing the .js file is also a tough task. Missing a lot of natural features.
Smart lookup is a feature where the Bing search engine is bundled in the app to allow users to automatically search for relevant websites, information and images based on the word highlighted by users, without need to open external browsers. The results will be shown on the right pane of the windo...
Microsoft Outlook comes with feature called "Clutter" to filter out incoming email based on the priority of the message set by user. User will be alerted when the high-priority message is incoming, whereas the low-priority message will be put in a separate folder.
Mail and Calendar support multiple accounts from multiple services, including Gmail and Google Apps. Gmail and Google Apps support was not available on Windows 8. It is available on the Windows 10 version of Mail and Calendar app.
Mailbird does collect some user data such as user email addresses, names and client use. So for those that are security minded the use collection of this information may not be wanted and can not be avoided.
Developers changed their business model to the subscription based completely unanounced. Consequently, customers who hadn't logged in for a while could no longer access their account and remove their data. HQ rarely answer emails from users who have issues.
Seamonkey is more than just an e-mail client as it is a full featured web browser as well. This makes for a good app to use for those that dislike having many different apps on their devices as it is a bit like an all in one solution.