Well, Samsung Email is definitely one popular app. Even though it works only on devices with Android 7.0 or higher, and most of these devices already have it pre-installed, Samsung Email has still managed to get downloaded one hundred million times since its release in January 2017. Part of the reason for such a high number is probably the fact that the app is updated not via an OTA but through the Play Store. Another reason is the fact that Samsung Email supports IMAP, POP3 and Exchange ActiveSync, which means that it can contain all of your e-mails.
Want to check it out? You can download the Samsung Email from here.