Note: The smtp-library is no longer maintained by me. You can get the current version at http://wiki.wxwidgets.org/wiki.pl?WxSMTP.
wxWindows is an open source C++ GUI framework for developing for Microsoft Windows, Mac, Unix and some other platforms. The framework hides the platform specific programming, so you can write one program and compile it on every supported system with nearly no changes.
I tried the framework because I didn't found a good GUI framework for Mac OS and I want an easy way to port it to other platforms, if I need to release it on more platforms than Mac OS. wxWindows solved this requirements and the layout-manger concept (wxBoxSizer etc., looks a bit like the layout-mangaer concept in Java) is a very good solution, specially for creating multi-language applications.
Because wxWindows has no SMTP implementation, I've done something by myself: Wino.
This was my first application with wxWindows: AddressManager.