You're not linking your project with the qxmpp library. I don't have any idea how it can compile even under linux without having the LIBS += ...
line present.
My bet is: you've cut out the LIBS
line, and that line is left from the Linux compilation, and doesn't make sense on Windows. You need to adjust it just like you did for INCLUDEPATH.
You may also want to have separate settings for both platforms, so that you don't have to continuously tweak the project file:
win32 {
INCLUDEPATH += ...
LIBS += ...
}
unix {
INCLUDEPATH += ...
LIBS += ...
}