@Phataas It looks like you need the SMTP Client as a shared library compiled for the Android target architecture (it seems armv7 per your post). So step 1 is to build the SMTP Client as library for armv7. You're very likely need a cross-compile environment under Linux and installing the Android NDK, Once you have the resulting .so library, you need to put it in folder android/libs/armeabi-v7a under your project root folder, so the library file is deployed to the Android device along all other files.
I've been using an external library with my Android app without any issues, given creating the .so file was straitghforward. See project PixelKnot about how they use the external library F5Android
Good luck!