The stock Mail app in iOS is the default mail client and there’s no way that you can change this. This means that you’re stuck with using the stock Mail app for things like composing an email when clicking on mail hyperlinks such as ‘mail:to’.
There are tons of amazing third-party mail apps in the App Store that offer interesting features and if you ever wished to use any of these as your default mail app, then check out a new jailbreak tweak known as MailClientDefault10. With this tweak, you can easily set a third-party email app as your default mail client.
The tweak currently supports four third-party mail apps including:
Once you install MailClientDefault10, head to its preferences pane in Settings and there you can select a default mail client from the four supported email apps. Once selected, whenever your iPhone attempts to open the default mail app, it will open the third-party app that you chose in Settings rather than the stock Mail app.
Apart from these four email apps, there are tons of other amazing email clients which I wish the tweak supported. I believe the developer can easily add support for new email apps and we are hopeful that future updates will bring a more extensive list of supported mail clients.
If you wish to give this tweak a try, it is available for free in Cydia’s BigBoss repo and supports iOS 10. Let us know what you think about it once you give it a go.