Well, I'm contributing only to the Open Source parts, under LGPL and GPL like licences. No need to reverse engineer anything, all the code is with free software licences !

• The low level email handling is done via QMF (Qt Messaging Framework), so the main work is going there, by adding cryptographic capabilities through a new plugin framework, implementing OpenPGP signature at the moment but being ready for encryption or other cryptographic standards.
• Then there are the QML bindings for it (nemo-qml-plugin-email) which is open source also.
• Additionally, some new code to add QML bindings for GPG operations (like key listing), open source also.
• And finally, the only part that goes to closed source parts (but consist of ten to twenty lines of QML text maximum) is the addition of some buttons in the UI, given as a patch to the official email client, so readable also, provided without any licence.

Conclusion everything is reusable out of the box without reverse engineering.